Materials to practical classes 5
MORPHOLOGY OF VEGETATIVE ORGANS OF PLANTS
Plant morphology (or phytomorphology) is the general term for the study of the morphology (physical form and external structure) of plants.[1] This is usually considered distinct from plant anatomy, which is the study of the internal structure of plants, especially at the microscopic level. Plant morphology is useful in the identification of plants.
Plant morphology “represents a study of the development, form, and structure of plants, and, by implication, an attempt to interpret these on the basis of similarity of plan and origin.”[2] There are four major areas of investigation in plant morphology, and each overlaps with another field of the biological sciences.
First of all, morphology is comparative, meaning that the morphologist examines structures in many different plants of the same or different species, then draws comparisons and formulates ideas about similarities. When structures in different species are believed to exist and develop as a result of common, inherited genetic pathways, those structures are termed homologous. For example, the leaves of pine, oak, and cabbage all look very different, but share certain basic structures and arrangement of parts. The homology of leaves is an easy conclusion to make. The plant morphologist goes further, and discovers that the spines of cactus also share the same basic structure and development as leaves in other plants, and therefore cactus spines are homologous to leaves as well. This aspect of plant morphology overlaps with the study of plant evolution and paleobotany.
A comparative science
A plant morphologist makes comparisons between structures in many different plants of the same or different species. Making such comparisons between similar structures in different plants tackles the question of why the structures are similar. It is quite likely that similar underlying causes of genetics, physiology, or response to the environment have led to this similarity in appearance. The result of scientific investigation into these causes can lead to one of two insights into the underlying biology:
1. Homology – the structure is similar between the two species because of shared ancestry and common genetics.
2. Convergence – the structure is similar between the two species because of independent adaptation to common environmental pressures.
Understanding which characteristics and structures belong to each type is an important part of understanding plant evolution. The evolutionary biologist relies on the plant morphologist to interpret structures, and in turn provides phylogenies of plant relationships that may lead to new morphological insights.
Homology
When structures in different species are believed to exist and develop as a result of common, inherited genetic pathways, those structures are termed homologous. For example, the leaves of pine, oak, and cabbage all look very different, but share certain basic structures and arrangement of parts. The homology of leaves is an easy conclusion to make. The plant morphologist goes further, and discovers that the spines of cactus also share the same basic structure and development as leaves in other plants, and therefore cactus spines are homologous to leaves as well.
Convergence
When structures in different species are believed to exist and develop as a result of common adaptive responses to environmental pressure, those structures are termed convergent. For example, the fronds of Bryopsis plumosa and stems of Asparagus setaceus both have the same feathery branching appearance, even though one is an alga and one is a flowering plant. The similarity in overall structure occurs independently as a result of convergence. The growth form of many cacti and species of Euphorbia is very similar, even though they belong to widely distant families. The similarity results from common solutions to the problem of surviving in a hot, dry environment.
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Astrophytum asterias, a cactus. |
Vegetative and reproductive characters
Plant morphology treats both the vegetative structures of plants, as well as the reproductive structures.
The vegetative (somatic) structures of vascular plants include two major organ systems: (1) a shoot system, composed of stems and leaves, and (2) a root system. These two systems are common to nearly all vascular plants, and provide a unifying theme for the study of plant morphology.
By contrast, the reproductive structures are varied, and are usually specific to a particular group of plants. Structures such as flowers and fruits are only found in the angiosperms; sori are only found in ferns; and seed cones are only found in conifers and other gymnosperms. Reproductive characters are therefore regarded as more useful for the classification of plants than vegetative characters.
When characters are used in descriptions or for identification they are called diagnostic or key characters which can be either qualitative and quantitative.
1. Quantitative characters are morphological features that can be counted or measured for example a plant species has flower petals 10-
2. Qualitative characters are morphological features such as leaf shape, flower color or pubescence.
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The detailed study of reproductive structures in plants led to the discovery of the alternation of generations, found in all plants and most algae, by the German botanist Wilhelm Hofmeister. This discovery is one of the most important made in all of plant morphology, since it provides a common basis for understanding the life cycle of all plants.
Plant development
Plant development is the process by which structures originate and mature as a plant grows. It is a subject studies in plant anatomy and plant physiology as well as plant morphology.
The process of development in plants is fundamentally different from that seen in vertebrate animals. When an animal embryo begins to develop, it will very early produce all of the body parts that will ever have in its life. When the animal is born. or hatches from its egg. it has all its body parts and from that point will only grow larger and more mature. By contrast, plants constantly produce new tissues and structures throughout their life from meristems located at the tips of organs, or between mature tissues. Thus, a living plant always has embryonic tissues.
The properties of organization seen in a plant are emergent properties which are more than the sum of the individual parts. “The assembly of these tissues and functions into an integrated multicellular organism yields not only the characteristics of the separate parts and processes but also quite a new set of characteristics which would not have been predictable on the basis of examination of the separate parts.”[4] In other words, knowing everything about the molecules in a plant are not enough to predict characteristics of the cells; and knowing all the properties of the cells will not predict all the properties of a plant’s structure.
Root
In vascular plants, the root is the organ of a plant body that typically lies below the surface of the soil. But, this is not always the case, since a root can also be aerial (that is, growing above the ground) or aerating (that is, growing up above the ground or especially above water). On the other hand, a stem normally occurring below ground is not exceptional either (see rhizome). So, it is better to define root as a part of a plant body that bears no leaves, and therefore also lacks nodes. There are also important internal structural differences between stems and roots. The two major functions of roots are 1.) absorption of water and inorganic nutrients and 2.) anchoring the plant body to the ground. Roots also function in cytokinin synthesis, which supplies some of the shoot’s needs. They often function in storage of food. The roots of most vascular plant species enter into symbiosis with certain fungi to form mycorrhizas, and a large range of other organisms including bacteria also closely associate with roots.
Roots of a hydroponically grown plant
At the tip of every growing root is a conical covering of tissue called the root cap, which consists of undifferentiated soft tissue (parenchyma) with unthickened walls covering the apical meristem. The root cap provides mechanical protection to the meristem as the root advances through the soil. As the root cap cells are worn away they are continually replaced by new cells generated by cell division within the meristem. The root cap is also involved in the production of mucigel, a sticky mucilage that coats the new formed cells. These cells contain statoliths, starch grains that move in response to gravity and thus control root orientation.
The outside surface of the primary root is the epidermis. Recently produced epidermal cells absorb water from the surrounding environment and produce outgrowths called root hairs that greatly increase the cell’s absorptive surface. Root hairs are very delicate and generally short-lived, remaining functional for only a few days. However, as the root grows, new epidermal cells emerge and these form new root hairs, replacing those that die. The process by which water is absorbed into the epidermal cells from the soil is known as osmosis. For this reason, water that is saline is more difficult for most plant species to absorb.
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Cross section of the root of a dicotyledon
Beneath the epidermis is the cortex, which comprises the bulk of the primary root. Its main function is storage of starch. Intercellular spaces in the cortex aerate cells for respiration. An endodermis is a thin layer of small cells forming the innermost part of the cortex and surrounding the vascular tissues deeper in the root. The tightly packed cells of the endodermis contain a substance known as suberin in their cell walls. This suberin layer is the Casparian strip, which creates an impermeable barrier of sorts. Mineral nutrients can only move passively within root cell walls until they reach the endodermis. At that point, they must be actively transported across a cell membrane to continue further into the root. This allows the plant to accumulate mineral nutrients in the stele.
The vascular cylinder, or stele, consists of the cells inside the endodermis. The outer part, known as the pericycle, surrounds the actual vascular tissue. In monocotyledonous plants, the xylem and phloem cells are arranged in a circle around a pith or center, whereas in dicotyledons, the xylem cells form a central “hub” with lobes, and phloem cells fill in the spaces between the lobes.
All roots have primary growth or growth in length. Roots of many vascular plants, especially dicots and gymnosperms, often undergo secondary growth, which is an increase in diameter. A vascular cambium forms in the stele to produce secondary phloem and secondary xylem. The epidermis is replaced by a periderm. As the stele increases in diameter, the cortex, pericycle and endodermis are lost. Eveon-woody roots often undergo secondary growth, including those of tomato and alfalfa.
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Early root growth is one of the functions of the apical meristem located near the tip of the root. The meristem cells more or less continuously divide, producing more meristem, root cap cells (these sacrificed to protect the meristem), and undifferentiated root cells. The latter will become the primary tissues of the root, first undergoing elongation, a process that pushes the root tip forward in the growing medium. Gradually these cells differentiate and mature into specialized cells of the root tissues.
Roots will generally grow in any direction where the correct environment of air, mineral nutrients and water exists to meet the plant’s needs. Roots will not grow in dry soil. Over time, given the right conditions, roots can crack foundations, snap water lines, and lift sidewalks. At germination, roots grow downward due to gravitropism, the growth mechanism of plants that also causes the shoot to grow upward. In some plants (such as ivy), the “root” actually clings to walls and structures.
Growth from apical meristems is known as primary growth, which encompasses all elongation. Secondary growth encompasses all growth in diameter, a major component of woody plant tissues and many nonwoody plants. For example, storage roots of sweet potato have secondary growth but are not woody. Secondary growth occurs at the lateral meristems, namely the vascular cambium and cork cambium. The former forms secondary xylem and secondary phloem, while the latter forms the periderm.
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In plants with secondary growth, the vascular cambium, originating between the xylem and the phloem, forms a cylinder of tissue along the stem and root. The cambium layer forms new cells on both the inside and outside of the cambium cylinder, with those on the inside forming secondary xylem cells, and those on the outside forming secondary phloem cells. As secondary xylem accumulates, the “girth” (lateral dimensions) of the stem and root increases. As a result, tissues beyond the secondary phloem (including the epidermis and cortex, in many cases) tend to be pushed outward and are eventually “sloughed off” (shed).
Stilt roots in the Amazon Rainforest support a tree in very soft, wet soil conditions
The vascular cambium produces new layers of secondary xylem annually. The xylem vessels are dead at maturity but are responsible for most water transport through the vascular tissue in stems and roots.
A true root system consists of a primary root and secondary roots (or lateral roots).
The primary root originates in the radicle of the seedling. It is the first part of the root to be originated. During its growth it rebranches to form the lateral roots. It usually grows downwards. Generally, two categories are recognized:
- the taproot system: the primary root is prominent and has a single, dominant axis; there are fibrous secondary roots running outward. Usually allows for deeper roots capable of reaching low water tables. Most common in dicots. The main function of the taproot is to store food.
- the diffuse root system: the primary root is not dominant; the whole root system is fibrous and branches in all directions. Most common in monocots. The main function of the fibrous root is to anchor the plant.
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Aerating roots of a mangrove
The roots, or parts of roots, of many plant species have become specialized to serve adaptive purposes besides the two primary functions described in the introduction.
- Adventitious roots arise out-of-sequence from the more usual root formation of branches of a primary root, and instead originate from the stem, branches, leaves, or old woody roots. They commonly occur in monocots and pteridophytes, but also in many dicots, such as clover (Trifolium), ivy (Hedera), strawberry (Fragaria) and willow (Salix). Most aerial roots and stilt roots are adventitious. In some conifers adventitious roots can form the largest part of the root system.
- Aerating roots (or pneumatophores): roots rising above the ground, especially above water such as in some mangrove genera (Avicennia, Sonneratia). In some plants like Avicennia the erect roots have a large number of breathing pores for exchange of gases.
- Aerial roots: roots entirely above the ground, such as in ivy (Hedera) or in epiphytic orchids. They function as prop roots, as in maize or anchor roots or as the trunk in strangler fig.
- Contractile roots: they pull bulbs or corms of monocots, such as hyacinth and lily, and some taproots, such as dandelion, deeper in the soil through expanding radially and contracting longitudinally. They have a wrinkled surface.
- Coarse roots: Roots that have undergone secondary thickening and have a woody structure. These roots have some ability to absorb water and nutrients, but their main function is transport and to provide a structure to connect the smaller diameter, fine roots to the rest of the plant.
- Fine roots: Primary roots usually <
2 mm diameter that have the function of water and nutrient uptake. They are often heavily branched and support mycorrhizas. These roots may be short lived, but are replaced by the plant in an ongoing process of root ‘turnover’. - Haustorial roots: roots of parasitic plants that can absorb water and nutrients from another plant, such as in mistletoe (Viscum album) and dodder.
- Propagative roots: roots that form adventitious buds that develop into aboveground shoots, termed suckers, which form new plants, as in Canada thistle, cherry and many others.
- Proteoid roots or cluster roots: dense clusters of rootlets of limited growth that develop under low phosphate or low iron conditions in Proteaceae and some plants from the following families Betulaceae, Casuarinaceae, Eleagnaceae, Moraceae, Fabaceae and Myricaceae.
- Stilt roots: these are adventitious support roots, common among mangroves. They grow down from lateral branches, branching in the soil.
- Storage roots: these roots are modified for storage of food or water, such as carrots and beets. They include some taproots and tuberous roots.
- Structural roots: large roots that have undergone considerable secondary thickening and provide mechanical support to woody plants and trees.
- Surface roots: These proliferate close below the soil surface, exploiting water and easily available nutrients. Where conditions are close to optimum in the surface layers of soil, the growth of surface roots is encouraged and they commonly become the dominant roots.
- Tuberous roots: A portion of a root swells for food or water storage, e.g. sweet potato. A type of storage root distinct from taproot.
Aerial roots are roots that are aboveground. They are almost always adventitious. They are found in diverse plant species, including epiphytes also known as air plants, which includes the orchids, tropical coastal swamp trees such as mangroves, the resourceful banyan tree, the warm-temperate rainforest rātā and pōhutukawa trees of New Zealand and vines like English ivy and irritating poison ivy.
This plant organ that is found in so many diverse plant families has different specializations that suit the plant habitat. In general growth form, they can be technically classed as negatively gravitropic (grows up and away from the ground) or positively gravitropic (grows down toward the ground).
Non-parasitic ivy are vines that use their aerial roots to cling to host plants, rocks, or houses. Prop roots form on aerial stems and grow down into the soil to brace the plant, e.g. maize and screw pine.
The Banyan tree (Ficus sp.) is an example of a strangler fig that begins life as an epiphyte in the crown of another tree. Its roots grow down and around the stem of the host, their growth accelerating once the ground has been reached. Over time, the roots coalesce to form a pseudotrunk, eventually strangling and killing the host. Another strangler that begins life as an epiphyte is the Moreton Bay Fig {Ficus macrophylla) of tropical and subtropical eastern
These specialized aerial roots enable plants to breathe air in habitats that have waterlogged soil. The roots may grow down from the stem, or up from typical roots. Some botanists classify these as aerating roots rather than aerial roots, if they come up from soil. The surface of these roots are covered with lenticels which take up air into spongy tissue which in turn uses osmotic pathways to spread oxygen throughout the plant as needed.
Black mangrove is differentiated from other mangrove species by its pneumatophores.
See also Cypress knee
These roots are found in parasitic plants, where aerial roots become cemented to the host plant via a sticky attachment disc before intruding into the tissues of the host. Mistletoe is a good example of this.
Horizontal, aboveground stems, termed stolons or runners, usually develop plantlets with adventitious roots at their nodes, e.g. strawberry and spider plant.
Some leaves develop adventitious buds, which then form adventitious roots, e.g. piggyback plant (Tolmiea menziesii) and mother-of-thousands (Kalanchoe daigremontiana). The adventitious plantlets then drop off the parent plant and develop as separate clones of the parent.
Plant stem
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Stem showing internode and nodes plus leaf petiole and new stem rising from node.
A stem is one of two main structural axes of a vascular plant. The stem is normally divided into nodes and internodes, the nodes hold buds which grow into one or more leaves, inflorescence (flowers), cones or other stems etc. The internodes act as spaces that distance one node from another. The term shoots is often confused with stems; shoots generally refer to new fresh plant growth and does include stems but also to other structures like leaves or flowers. The other main structural axis of plants is the root. In most plants stems are located above the soil surface but some plants have underground stems.
Stems have four main functions which are:
- Support for and the elevation of leaves, flowers and fruits. The stems keep the leaves in the light and provide a place for the plant to keep its flowers and fruits.
- Transport of fluids between the roots and the shoots in the xylem and phloem.
- Storage of nutrients.
- The production of new living tissue. The normal life span of plant cells is one to three years. Stems have cells called meristems that annually generate new living tissue.
Stem showing internode and nodes plus leaf petioles.
Stems are often specialized for storage, asexual reproduction, protection or photosynthesis, including the following:
- Acaulescent – plants with very short stems that appear to have no stems. The leaves appear to rise out of the ground, e.g. some Viola.
- Arborescent – tree like with woody stems normally with a single trunk.
- Bud – an embryonic shoot with immature stem tip.
- Bulb – a short vertical underground stem with fleshy storage leaves attached, e.g. onion, daffodil, tulip. Bulbs often function in reproduction by splitting to form new bulbs or producing small new bulbs termed bulblets. Bulbs are a combination of stem and leaves so may better be considered as leaves because the leaves make up the greater part.
- Caespitose – when stems grow in a tangled mass or clump or in low growing mats.
- Cladophyll – a flattened stem that appears leaf like and is specialized for photosynthesis, e.g. asparagus, cactus pads.
- Climbing – stems that cling or wrap around other plants or structures.
- Corm – a short enlarged underground, storage stem, e.g. taro, crocus, gladiolus.
- Decumbent – stems that lay flat on the ground and turn upwards at the ends.
- Fruticose – stems that grow shrub like with woody like habit.
- Herbaceous – non woody, they die at the end of the growing season.
- Pseudostem – A false stem made of the rolled bases of leaves, which may be 2 or
3 m tall as in banana - Rhizome – a horizontal underground stem that functions mainly in reproduction but also in storage, e.g. most ferns, iris
- Runner (plant part) – a type of stolon, horizontally growing on top of the ground and rooting at the nodes. e.g. strawberry, spider plant.
- Scape – a stem that holds flowers that comes out of the ground and has no normal leaves. Hosta, Lily, Iris.
- Stolons – a horizontal stem that produces rooted plantlets at its nodes and ends, forming near the surface of the ground.
- Tree – a woody stem that is longer than
5 meters with a main trunk. - Thorns – a reduced stem with a sharp point and rounded shape. e.g. honey locust, hawthorn.
- Tuber – a swollen, underground storage stem adapted for storage and reproduction, e.g. potato.
- Woody – hard textured stems with secondary xylem.
White and green asparagus – crispy stems are the edible parts of this vegetable
There are thousands of species whose stems have economic uses. Stems provide a few major staple crops such as potato and taro. Sugar cane stems are a major source of sugar. Maple sugar is obtained from trunks of maple trees. Vegetables from stems are asparagus, bamboo shoots, cactus pads or nopalitos, kohlrabi, and water chestnut. The spice, cinnamon is bark from a tree trunk. Cellulose from tree trunks is a food additive in bread, grated Parmesan cheese, and other processed foods. Gum arabic is an important food additive obtained from the trunks of Acacia senegal trees. Chicle, the main ingredient in chewing gum, is obtained from trunks of the chicle tree.
Medicines obtained from stems include quinine from the bark of cinchona trees, camphor distilled from wood of a tree in the same genus that provides cinnamon, and the muscle relaxant curare from the bark of tropical vines.
Wood is a used in thousands of ways, e.g. buildings, furniture, boats, airplanes, wagons, car parts, musical instruments, sports equipment, railroad ties, utility poles, fence posts, pilings, toothpicks, matches, plywood, coffins, shingles, barrel staves, toys, tool handles, picture frames, veneer, charcoal and firewood. Wood pulp is widely used to make paper, cardboard, cellulose sponges, cellophane and some important plastics and textiles, such as cellulose acetate and rayon. Bamboo stems also have hundreds of uses, including paper, buildings, furniture, boats, musical instruments, fishing poles, water pipes, plant stakes, and scaffolding. Trunks of palm trees and tree ferns are often used for building. Reed stems are also important building materials in some areas.
Tannins used for tanning leather are obtained from the wood of certain trees, such as quebracho. Cork is obtained from the bark of the cork oak. Rubber is obtained from the trunks of Hevea brasiliensis. Rattan, used for furniture and baskets, is made from the stems of tropical vining palms. Bast fibers for textiles and rope are obtained from stems include flax, hemp, jute and ramie. The earliest paper was obtained from the stems of papyrus by the ancient Egyptians.
Amber is fossilized sap from tree trunks; it is used for jewelry and may contain ancient animals. Resins from conifer wood are used to produce turpentine and rosin. Tree bark is often used as a mulch
and in growing media for container plants.
Some ornamental plants are grown mainly for their attractive stems, e.g.:
Branch
A branch (American English IPA: /ˈbræntʃ/, British English IPA: /ˈbrɑːntʃ/) or tree branch (sometimes referred to in botany as a ramus) is a woody structural member connected to but not part of the central trunk of a tree (or sometimes a shrub). Large branches are known as boughs and small branches are known as twigs.
Cork – an external, secondary tissue impermeable to water and gases. - Cork cambium – A layer of cells, normally one or two cell layers thick that is in a persistent meristematic state that produces cork.
- Phelloderm – (not always present) A layer of cells formed in some plants from the inner cells of the cork cambium (
Cork - Cortex – The primary tissue of stems and roots. In stems the cortex is between the epidermis layer and the phloem, in roots the inner layer is not phloem but the pericycle.
- Phloem – nutrient-conducting tissue composed of sieve tube or sieve cells mixed with parenchyma and fibers.
Definitions of the term can vary. In another usage, bark consists of the dead and protective tissue found on the outside of a woody stem, and does not include the vascular tissue. The vascular cambium is the only part of a woody stem where cell division occurs. It contains undifferentiated cells that divide rapidly to produce secondary xylem to the inside and secondary phloem to the outside.
MODIFICATIONS OF SHOOT
Bulb
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A bulb is an underground vertical shoot that has modified leaves (or thickened leaf bases) that are used as food storage organs by a dormant plant.
A bulb’s leaf bases generally do not support leaves, but contain food reserves to enable the plant to survive adverse conditions. The leaf bases may resemble scales, or they may overlap and surround the center of the bulb as with the onion. A modified stem forms the base of the bulb, and plant growth occurs from this basal plate. Roots emerge from the underside of the base, and new stems and leaves from the upper side.
Other types of storage organs (such as corms, rhizomes, and tubers) are sometimes erroneously referred to as bulbs. The correct term for plants that form underground storage organs, including bulbs as well as tubers and corms, is geophyte. Some epiphytic orchids (family Orchidaceae) form above-ground storage organs called pseudobulbs, that superficially resemble bulbs.
Plants that form true bulbs are all monocotyledons, and include:
- Onion, garlic, and other alliums, family Alliaceae.
- Lily, tulip, and many other members of the lily family Liliaceae.
- Amaryllis, Hippeastrum, Narcissus, and several other members of the amaryllis family Amaryllidaceae.
- Two groups of Iris species, family Iridaceae: subgenus Xiphium (the “Dutch” irises) and subgenus Hermodactyloides (the miniature “rock garden” irises).
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Some lilies form small bulbs, called bulbils in their leaf axils. Several members of the onion family, Alliaceae, including Allium sativum (garlic), form bulbils in their flower heads, sometimes as the flowers fade, or even instead of the flowers. The so-called Tree onion (Allium cepa var. proliferum) forms small onions which are large enough for pickling.
Some ferns, such as Hen and Chicken Fern grow offshoots on top of their fronds, which are also referred to as bulbils.
Rhizome
Ginger rhizome
In botany, a rhizome is a horizontal stem of a plant that is usually found underground, often sending out roots and shoots from its nodes. Plants with underground rhizomes include ginger, hops, and turmeric, significant for their medicinal properties, and the weeds Johnson grass, bermuda grass, and purple nut sedge. Some plants have rhizomes that grow above ground or that sit at the soil surface, including some Iris species, and ferns, whose spreading stems are rhizomes. Rhizomes may also be referred to as creeping rootstalks, or rootstocks. A stolon is similar to a rhizome, but, unlike a rhizome, which is the main stem of the plant, a stolon sprouts from an existing stem, has long internodes, and generates new shoots at the end, e.g., the strawberry plant. In general, rhizomes have short internodes; they send out roots from the bottom of the nodes and new upward-growing shoots from the top of the nodes.
For many plants, the rhizome is used by gardeners to propagate the plants by a process known as vegetative reproduction. Examples of plants that are propagated this way include asparagus, ginger, irises, Lily of the Valley, Cannas, and sympodial orchids.
A stem tuber is a thickened part of a rhizome or stolon that has been enlarged for use as a storage organ. [1] In general, a tuber is high in starch, for example, the common potato, which is a modified stolon. The term tuber is often used imprecisely, and is sometimes applied to plants with rhizomes.
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The rhizome is a key metaphor in the philosophy of Gilles Deleuze and Felix Guattari: see rhizome (philosophy).
Leaf
In botany, a leaf is an above-ground plant organ specialized for photosynthesis. For this purpose, a leaf is typically flat (laminar) and thin, to expose the cells containing chloroplast to light over a broad area, and to allow light to penetrate fully into the tissues. Leaves are also the sites in most plants where transpiration and guttation take place. Leaves can store food and water, and are modified in some plants for other purposes. The comparable structures of ferns are correctly referred to as fronds. Furthermore, leaves are prominent in the human diet as leaf vegetables.
Autumn Leaves
Fallen autumn leaves
Leaves in temperate, boreal, and seasonally dry zones may be seasonally deciduous (falling off or dying for the inclement season). This mechanism to shed leaves is called abscission. After the leaf is shed, a leaf scar develops on the twig. In cold autumns they sometimes change color, and turn yellow, bright orange or red as various accessory pigments (carotenoids and xanthophylls) are revealed when the tree responds to cold and reduced sunlight by curtailing chlorophyll production. Red anthocyanin pigments are now thought to be produced in the leaf as it dies, possibly to mask the yellow hue left when the chlorophyll is lost – yellow leaves appear to attract herbivores such as aphids.[1]
External leaf characteristics (such as shape, margin, hairs, etc.) are important for identifying plant species, and botanists have developed a rich terminology for describing leaf characteristics. These structures are a part of what makes leaves determinant; they grow and achieve a specific pattern and shape, then stop. Other plant parts like stems or roots are non-determinant, and will usually continue to grow as long as they have the resources to do so.
Classification of leaves can occur through many different designative schema, and the type of leaf is usually characteristic of a species, although some species produce more than one type of leaf. The longest type of leaf is a leaf from palm trees, measuring at nine feet long. The terminology associated with the description of leaf morphology is presented, in illustrated form
Basic leaf types
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Leaves of the White Spruce (Picea glauca) are needle-shaped and their arrangement is spiral
- Ferns have fronds.
- Conifer leaves are typically needle-, awl-, or scale-shaped
- Angiosperm (flowering plant) leaves: the standard form includes stipules, a petiole, and a lamina.
- Lycophytes have microphyll leaves.
- Sheath leaves (type found in most grasses).
- Other specialized leaves (such as those of Nepenthes)
Different terms are usually used to describe leaf placement (phyllotaxis):
The leaves on this plant are arranged in pairs opposite one another, with successive pairs at right angles to each other (“decussate”) along the red stem. Note developing buds in the axils of these leaves.
- Alternate — leaf attachments are singular at nodes, and leaves alternate direction, to a greater or lesser degree, along the stem.
- Opposite — leaf attachments are paired at each node; decussate if, as typical, each successive pair is rotated 90° progressing along the stem; or distichous if not rotated, but two-ranked (in the same geometric flat-plane).
- Whorled — three or more leaves attach at each point or node on the stem. As with opposite leaves, successive whorls may or may not be decussate, rotated by half the angle between the leaves in the whorl (i.e., successive whorls of three rotated 60°, whorls of four rotated 45°, etc). Opposite leaves may appear whorled near the tip of the stem.
- Rosulate — leaves form a rosette
As a stem grows, leaves tend to appear arranged around the stem in a way that optimizes yield of light. In essence, leaves form a helix pattern centred around the stem, either clockwise or counterclockwise, with (depending upon the species) the same angle of divergence. There is a regularity in these angles and they follow the numbers in a Fibonacci sequence: 1/2, 2/3, 3/5, 5/8, 8/13, 13/21, 21/34, 34/55, 55/89. This series tends to a limit of 360° x 34/89 = 137.52 or 137° 30′, an angle known mathematically as the golden angle. In the series, the numerator indicates the number of complete turns or “gyres” until a leaf arrives at the initial position. The denominator indicates the number of leaves in the arrangement. This can be demonstrated by the following:
- alternate leaves have an angle of 180° (or 1/2)
- 120° (or 1/3) : three leaves in one circle
- 144° (or 2/5) : five leaves in two gyres
- 135° (or 3/8) : eight leaves in three gyres.
Divisions of the lamina (blade)
Two basic forms of leaves can be described considering the way the blade is divided. A simple leaf has an undivided blade. However, the leaf shape may be formed of lobes, but the gaps between lobes do not reach to the main vein. A compound leaf has a fully subdivided blade, each leaflet of the blade separated along a main or secondary vein. Because each leaflet can appear to be a simple leaf, it is important to recognize where the petiole occurs to identify a compound leaf. Compound leaves are a characteristic of some families of higher plants, such as the Fabaceae. The middle vein of a compound leaf or a frond, when it is present, is called a rachis.
- Palmately compound leaves have the leaflets radiating from the end of the petiole, like fingers off the palm of a hand, e.g. Cannabis (hemp) and Aesculus (buckeyes).
- Pinnately compound leaves have the leaflets arranged along the main or mid-vein.
- odd pinnate: with a terminal leaflet, e.g. Fraxinus (ash).
- even pinnate: lacking a terminal leaflet, e.g. Swietenia (mahogany).
- Bipinnately compound leaves are twice divided: the leaflets are arranged along a secondary vein that is one of several branching off the rachis. Each leaflet is called a “pinnule“. The pinnules on one secondary vein are called “pinna“; e.g. Albizia (silk tree).
- trifoliate: a pinnate leaf with just three leaflets, e.g. Trifolium (clover), Laburnum (laburnum).
pinnatifid: pinnately dissected to the midrib, but with the leaflets not entirely separate, e.g. Polypodium, some Sorbus (whitebeams).
Characteristics of the petiole
The overgrown petioles of Rhubarb (Rheum rhabarbarum) are edible.
Petiolated leaves have a petiole. Sessile leaves do not: the blade attaches directly to the stem. In clasping or decurrent leaves, the blade partially or wholly surrounds the stem, often giving the impression that the shoot grows through the leaf. When this is actually the case, the leaves are called “perfoliate“, such as in Claytonia perfoliata. In peltate leaves, the petiole attaches to the blade inside from the blade margin.
In some Acacia species, such as the Koa Tree (Acacia koa), the petioles are expanded or broadened and function like leaf blades; these are called phyllodes. There may or may not be normal pinnate leaves at the tip of the phyllode.
A stipule, present on the leaves of many dicotyledons, is an appendage on each side at the base of the petiole resembling a small leaf. Stipules may be lasting and not be shed (a stipulate leaf, such as in roses and beans), or be shed as the leaf expands, leaving a stipule scar on the twig (an exstipulate leaf).
Venation (arrangement of the veins)
Palmate-veined leaf
There are two subtypes of venation, namely, craspedodromous, where the major veins stretch up to the margin of the leaf, and camptodromous, when major veins extend close to the margin, but bend before they intersect with the margin.
- Feather-veined, reticulate — the veins arise pinnately from a single mid-vein and subdivide into veinlets. These, in turn, form a complicated network. This type of venation is typical for (but by no means limited to) dicotyledons.
- Pinnate-netted, penniribbed, penninerved, penniveined; the leaf has usually one main vein (called the mid-vein), with veinlets, smaller veins branching off laterally, usually somewhat parallel to each other; eg Malus (apples).
- Three main veins branch at the base of the lamina and run essentially parallel subsequently, as in Ceanothus. A similar pattern (with 3-7 veins) is especially conspicuous in Melastomataceae.
- Palmate-netted, palmate-veined, fan-veined; several main veins diverge from near the leaf base where the petiole attaches, and radiate toward the edge of the leaf; e.g. most Acer (maples).
- Parallel-veined, parallel-ribbed, parallel-nerved, penniparallel — veins run parallel for the length of the leaf, from the base to the apex. Commissural veins (small veins) connect the major parallel veins. Typical for most monocotyledons, such as grasses.
- Dichotomous — There are no dominant bundles, with the veins forking regularly by pairs; found in Ginkgo and some pteridophytes.
Note that although it is the more complex pattern, branching veins appear to be plesiomorphic and in some form were present in ancient seed plants as long as 250 million years ago. A pseudo-reticulate venation that is actually a highly modified penniparallel one is an autapomorphy of some Melanthiaceae which are monocots, e.g. Paris quadrifolia (True-lover’s Knot).
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Leaf morphology changes within a single plant
- Homoblasty – Characteristic in which a plant has small changes in leaf size, shape, and growth habit between juvenile and adult stages.
- Heteroblasty – Charactistic in which a plant has marked changes in leaf size, shape, and growth habit between juvenile and adult stages.
The leaf margin is characteristic for a genus and aids in determining the species.
- entire: even; with a smooth margin; without toothing
- ciliate: fringed with hairs
- crenate: wavy-toothed; dentate with rounded teeth, such as Fagus (beech)
- dentate: toothed, such as Castanea (chestnut)
- coarse-toothed: with large teeth
- glandular toothed: with teeth that bear glands.
- denticulate: finely toothed
- doubly toothed: each tooth bearing smaller teeth, such as Ulmus (elm)
- lobate: indented, with the indentations not reaching to the center, such as many Quercus (oaks)
- palmately lobed: indented with the indentations reaching to the center, such as Humulus (hop).
- serrate: saw-toothed with asymmetrical teeth pointing forward, such as Urtica (nettle)
- serrulate: finely serrate
- sinuate: with deep, wave-like indentations; coarsely crenate, such as many Rumex (docks)
- spiny: with stiff, sharp points, such as some Ilex (hollies) and Cirsium (thistles).
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Leaves showing various morphologies. Clockwise from upper left: tripartite lobation, elliptic with serrulate margin, peltate with palmate venation, acuminate odd-pinnate (center), pinnatisect, lobed, elliptic with entire margin
- acuminate: long-pointed, prolonged into a narrow, tapering point in a concave manner.
- acute: ending in a sharp, but not prolonged point
- cuspidate: with a sharp, elongated, rigid tip; tipped with a cusp.
- emarginate: indented, with a shallow notch at the tip.
- mucronate: abruptly tipped with a small short point, as a continuation of the midrib; tipped with a mucro.
- mucronulate: mucronate, but with a smaller spine.
- obcordate: inversely heart-shaped, deeply notched at the top.
- obtuse: rounded or blunt
- truncate: ending abruptly with a flat end, that looks cut off.
- acuminate: coming to a sharp, narrow, prolonged point.
- acute: coming to a sharp, but not prolonged point.
- auriculate: ear-shaped
- cordate: heart-shaped with the notch towards the stalk.
- cuneate: wedge-shaped.
- hastate: shaped like an halberd and with the basal lobes pointing outward.
- oblique: slanting.
- reniform: kidney-shaped but rounder and broader than long.
- rounded: curving shape.
- sagittate: shaped like an arrowhead and with the acute basal lobes pointing downward.
- truncate: ending abruptly with a flat end, that looks cut off.
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Common Mullein (Verbascum thapsus) leaves are covered in dense, stellate trichomes.
Scanning electron microscope image of trichomes on the lower surface of a Coleus blumei (coleus) leaf.
“Hairs” on plants are properly called trichomes. Leaves can show several degrees of hairiness. The meaning of several of the following terms can overlap.
- glabrous: no hairs of any kind present.
- arachnoid, arachnose: with many fine, entangled hairs giving a cobwebby appearance.
- barbellate: with finely barbed hairs (barbellae).
- bearded: with long, stiff hairs.
- bristly: with stiff hair-like prickles.
- canescent: hoary with dense grayish-white pubescence.
- ciliate: marginally fringed with short hairs (cilia).
- ciliolate: minutely ciliate.
- floccose: with flocks of soft, woolly hairs, which tend to rub off.
- glandular: with a gland at the tip of the hair.
- hirsute: with rather rough or stiff hairs.
- hispid: with rigid, bristly hairs.
- hispidulous: minutely hispid.
- hoary: with a fine, close grayish-white pubescence.
- lanate, lanose: with woolly hairs.
- pilose: with soft, clearly separated hairs.
- puberulent, puberulous: with fine, minute hairs.
- pubescent: with soft, short and erect hairs.
- scabrous, scabrid: rough to the touch
- sericeous: silky appearance through fine, straight and appressed (lying close and flat) hairs.
- silky: with adpressed, soft and straight pubescence.
- stellate, stelliform: with star-shaped hairs.
- strigose: with appressed, sharp, straight and stiff hairs.
- tomentose: densely pubescent with matted, soft white woolly hairs.
- cano-tomentose: between canescent and tomentose
- felted-tomentose: woolly and matted with curly hairs.
- villous: with long and soft hairs, usually curved.
- woolly: with long, soft and tortuous or matted hairs.
In the course of evolution, leaves adapted to different environments in the following ways:
- A certain surface structure avoids moistening by rain and contaminations (Lotus effect).
- Sliced leaves reduce wind resistance.
- Hairs on the leaf surface trap humidity in dry climates and creates a large boundary layer and reduces water loss.
- Waxy leaf surfaces reduce water loss.
- Shiny leaves deflect the sun‘s rays.
- Reductions of leaf sizes accompanied by a transfer of the photosynthetic functions to the stems reduces water loss.
- In more or less opaque or buried in the soil leaves translucent windows filter the light before the photosynthetis takes place at the inner leaf surfaces (e.g. Fenestraria).
- Thicker leaves store water (leaf succulents).
- Aromatic oils, poisons or pheromones produced by leaf borne glands deter herbivores (e.g. eucalypts).
- Inclusions of crystalline minerals deters herbivores.
- A transformation into petals attracts pollinators.
- A transformation into spines protects the plants (e.g. cactus).
- A transformation into insect traps helps feeding the plants (carnivorous plants).
- A transformation into bulbs helps storing food and water (e.g. onion).
- A transformation into tendrils allow the plant to climb (e.g. pea).
A transformation into bracts and pseudanthia (false flowers) replaces normal flower structures if the true flowers are extremely reduced (e.g. Spurges).
Roots
Upon seed germination, the embryo root, called the radicle, grows and develops into the first root. The radicle may thicken into a taproot with many branching roots, or it may develop into many adventitious roots. The direct opposite of a taproot system is a fibrous root system. This develops out of the many adventitious roots. In diameter, the roots in a fibrous system are very fine. There are many mature plants that have a combination system, which means there is a main taproot with many branching fibrous roots attached. Root hairs, or extensions of the epidermis as explained in the plant tissue tutorial, significantly increase the contact surface area of the root system. This allows for more exchange with the surrounding soil.
In general, most dicot plants (peas, carrots), or two seed-leaf plants, have taproot systems while monocot plants (corn, grasses), or one seed-leaf plants, have fibrous root systems. Additional differences between dicots and monocots will be discussed later on.
Root Structure
Historically, developing roots have been categorized into four zones of development. These are not strict zones, but rather regions of cells that gradually develop into those of the next region. The zones vary widely as far as extent and levels of development.
Regions of root development:
1. Root cap
2. Region of cell division
3. Region of elongation
4. Region of maturation
We will discuss each region in greater detail.
Root cap
In some plants the root cap is quite large and obvious, while in others it is nearly impossible to find. The root cap is made of parenchyma cells that form a thimble shape, as a covering for the tip of each root. The cap serves several functions. The main function being protection as the delicate root tip pushes through soil particles. In the outer cells of the root cap, the golgi bodies secrete a slimy substance that lodges in the walls and eventually pass to the outside. As the cells slough off, replaced from the inside, they form a slimy lubricant that aids root tip movement through the soil. In addition, to aiding movement, the slime is a supportive medium for beneficial bacteria.
The root cap serves in an additional capacity in determining root growth direction. As the root cap has a life span of about one week, it can serve for some interesting experiments. Whether the cap sloughs off or is cut off, the root will grow in random directions, as opposed to downward, until a new root cap is formed. This lends support to the notion that the root cap functions in the perception of gravity. On the sides of the root cap amyloplasts, or plastids containing starch grains, collect facing the direction of gravitational force. In documented experiments, when the root is tipped horizontally from it’s vertical growing position, the amyloplasts will reshift themselves to the “bottom” of the cells in which they are found. In a short time or 30 minutes to a few hours the root will resume growing downward. While the exact nature of this gravitational response, or gravitropism, is not fully known, there is some evidence that the calcium ions found in amyloplasts does influence the distribution of growth hormones in plant cells.
The Region of Cell Division
The region of cell division is the next zone in from the root cap. The root cap arises from the cells in this zone. This inverted cup-shaped region is composed of an apical meristem at it’s edges. The cells divide every 12 to 36 hours at the tip of the meristem, while the ones at the base of the meristem may divide once every 200 to 500 hours. Interestingly enough, the divisions are rhythmic and peak usually twice a day around noon and midnight. In the interim the cells are not usually dividing. Most of the cells in this region are cube shaped with fairly large nuclei and few, if any, small vacuoles. As in stems as well, the apical meristem in the roots will subdivide and give rise to three meristematic areas: the protoderm, which gives rise to the epidermis; just to the inside of the protoderm, the ground meristem, which produces parenchyma cells of the cortex; and the solid looking cylinder in the center of the root, the procambium, which produces primary xylem and phloem. The central pith tissue is found in many monocots, such as grasses, but is generally not seen in mature dicot plants due to compression by the vascular cylinder.
The Region of Elongation
This region is merged with the upper (toward the soil surface), region of the root apical meristem. It is in this region that the cells become several times their original length, and somewhat wider. The tiny vacuoles in each cell will merge and become one or two large vacuoles. In their final state, the enlarged vacuoles will account for up to 90% or more of the cellular volume. As only the root cap and apical meristem are actually moving through the soil, no further increase in cell size occurs above the region of elongation. While the elongated portions of the root generally remain stationary for the rest of their life, if a cambium is present there may be secondary growth and an increase in root girth.
The Region of Maturation
The region of maturation is sometimes also called the region of differentiation or root-hair zone. In this region, cells mature into the various types of primary tissues. Recall that root hairs are extensions of the epidermis that serve to increase surface area and aid in absorption of water and soil nutrients. If the region of maturation is examined carefully, it would be noted that the cuticle is very thin on the root hairs and epidermal cells of roots. It is understood that any significant amount of fatty substance would interfere with the ability to absorb water, as fats are hydrophobic—or water repelling. A root in cross-section would have an epidermis, cortex, endodermis, xylem, phloem and a pericycle. The cortex is tissue at the immediate inside of the epidermis that functions in storing food. Generally, the cortex is many cells thick and similar to the cortex of stems, with the exception of the presence of a root endodermis at the inner boundary. In stems, an endodermis is quite rare, while in roots only three species of plants are known to lack a root endodermis. The endodermis is a cylinder formed by a single layer of tightly arranged cells. The primary walls of these cells contain suberin. The waterproof suberin forms bands, called Casparian strips, around the cell walls perpendicular to the root’s surface. The barrier that is formed forces all water and dissolved substances entering and leaving the central tissue core to pass through the plasma membrane or their plasmodesmata. This entire structure serves to regulate the types of minerals absorbed and transported by the root to the stems.
Next to the inside of the endodermis is a cylinder of parenchyma cells called the pericycle. The pericycle is generally one cell wide, however, it can extend for several cells depending on the plant. It is a vital tissue, as the pericycle is the point of origin for the lateral branch roots, and if it is a dicot, part of the vascular cambium. The cells in the pericycle retain their ability to divide even after they have matured. Primary xylem, which contains water conducting cells, forms at the core of the root and may or may not have observable ‘branches’ which extend like an ‘x’ to the pericycle. Primary phloem, which contains the food conducting cells, fills in the spaces between the branches of xylem. Any branch roots will usually arise in the pericycle opposite the xylem branches.
Most plants produce a fibrous root system, a taproot system, or most commonly a combination of both. However, some plants have roots with modifications that allow specific functions in addition to the absorption of water and minerals in solution.
Food-Storage Roots
In certain plants the roots, or part of the root system, is enlarged in order to store large quantities of starch and other carbohydrates. Sweet potatoes and yams, for example, have extra cambial cells that develop in the xylem portion of branch roots. The cambial cells produce numerous parenchyma cells that cause the organs to swell. Starches are then stored in the swollen areas of the root. Carrots, beets and turnips have storage organs that are actually a combination of root and stem. Approximately, the top two centimeters of a carrot are actually derived from the stem. Although, you likely will not be able to see the origin of the cells just by looking at a carrot.
Water-Storage Roots
Plants that grow in particularly arid regions are known for growing structures used to retain water. Some plants in the Pumpkin Family produce huge water storing roots. The plant will then use the stored water in times or seasons of low precipitation. Some cultures will harvest the water storing root and use them for drinking water. Plants storing up to 159 pounds (72 kilograms) of water in a single major root have been found and documented.
To propagate means to produce more of oneself. Propagative root structures are a way for a plant to produce more of itself. Adventitious buds are buds that appear in unusual places. Many plants will produce these buds along the roots that grow near the surface of the ground. Suckers, or aerial stems with rootlets, will develop from these adventitious buds. The ‘new’ plant can be separated from the original plant and can grow independently. Some plants will produce propagative roots up to 30 feet or more away from the parent plant. This can be a nuisance for some people, while others may enjoy the propagative qualities of their cherry tree, strawberries or horseradish plants.
Pneumatophores
Pneumatophores are spongy roots that develop in most plants that grow in water. Swamps, marshes and coastal areas are good places to find plants with pneumatophores. These specialized roots account for the fact that water, even after having air bubbled through it, has less than one thirtieth of the amount of free oxygen that is found in air. Plants growing in water may require additional methods of obtaining oxygen for respiration. Pneumatophores fill that need by rising above the water surface and facilitating gas exchange.
There are many different kinds of aerial roots produced by a wide variety of plants. Orchids produce velamen roots, corn plants have prop roots, ivies have adventitious roots and vanilla orchids even have photosynthetic roots that can manufacture food. Banyan trees have aerial roots that grow down from the tree branches until they touch find the soil. In a nutshell, aerial roots are roots that are not covered by soil hence out in the air. They can facilitate climbing and various types of support as demonstrated by ivies and creeper plants.
Contractile roots are roots that pull the plant deeper into the soil. Lily bulbs are a good example, as each bulb is pulled a little further into the soil as additional contractile roots are developed each year. When a region of stable temperature is reached, the contractile roots quit pulling. Dandelions also have contractile roots, and their presence is noticeable because the lower leaves may look like they are coming right out of the ground. In reality, the roots are pulling the stem downward. The actual mechanism of contraction involves the thickening and constriction of parenchyma cells. This causes the components of xylem to spiral into a corkscrew shape. The portion of the root that contracts may lose up to two-thirds of its length within weeks.
Tropical trees may have large buttress roots at the base of the trunk. These roots add stability to the tree and give an angular look to the lower visible portion of the trunk.
Some plants, such as dodders, broomrapes and pinedrops do not have chlorophyll. They will parasitize other plants and utilize their chlorophyll and food making abilities. The parasitic mechanism involves rootlike projections called haustoria (singular haustorium). These projections develop along stems that are in contact with the host. They will penetrate the outer tissues of the host plant, and will tap into the water and food conducting tissues (xylem and phloem). Other plants with chlorphyll, such as mistletoes, will also form haustoria in order to obtain water and dissolved minerals from host plants. They are capable of producing their own food, and thus are considered to be partially parasitic.
Mycorrhizae
Mycorrhizae are fungal roots found in many plants. These fungal associations are important for both the plant and for the fungal and are therefore considered to be mutualistic. Essentially, the fungus will have a greater capacity for absorbing phosphorus than root hairs alone. The fungus will also grow and increase the absorption of water and other nutrients. In return, the plant provides sugars and amino acids vital to the survival of the fungus. Plants with mycorrhizae generally have less root hairs than those without. Nearly all woody trees and shrubs found in forests have fungal associations in their root systems. However, it has been demonstrated that mycorrhizae are particularly susceptible to acid rain. This may have a direct impact on forest health and maintenance.
Root Nodules
It is important to note that root nodules are not root knots, which are root swellings in response to worm invasions. Root nodules are beneficial bacterial colonies that are visible as small swellings in the root system. The bacteria aid the plant in fixing, or converting, atmospheric nitrogen in to a form that the plant can use. Root nodules are found extensively throughout the legume family. A nodule develops when a substance leaked into the soil by plant roots stimulates Rhizobium bacteria to produce another substance that coused root hairs to bend sharply. The bacterium may attach in the crook of the bend and then ‘invade’ the cell with a tubular infection thread. This thread does not penetrate the cell wall and plasma membrane. The thread, does however, grow through to the cortex which is stimulated to produce new cells that will become part of the housing for the bacterium. As the bacteria multiply and the colony grows, the nodule will swell. It is in the crook of root hairs that the nitrogen fixing takes place.
External Form of a woody twig
A woody twig, or stem, is an axis with leaves attached. The leaves are arranged in various ways around and on the axis. You may hear them described as alternate, or alternately arranged, opposite or oppositely arranged, or if they are found in groups of three or more they may be referred to as whorled. The region, just a general area in this case, where the leaves attach to the stem are called nodes. The region of stem between two nodes is called the internode. The leaf blade is attached to the stem via a stalk called the petiole. In the angle, or axil, formed between the petiole and the stem you will find the axillary bud. Axillary refers to a structure that forms an armpit, just for trivia’s sake. These buds can become new branches or they may have tissues that will form into flowers for the next season. Most buds are protected by bud scales which fall off as bud tissue begins to grow. In general, at the tip of a twig a terminal (or ending) bud is present. It is larger than the axillary buds and produces tissues to extend twig length during the growing season. When the bud scales of a terminal bud fall off they leave scars on the twig. You can calculate the age of a twig by counting up the terminal bud scale scars. There are other scars on twigs that may look like terminal bud scars that are left by paired appendages called stipules which are found at the base of a petiole in the axil.
Trees and shrubs that lose their leaves every year, deciduous plants, have characteristic leaf scars with dormant, or not active, axillary buds directly above them. Sometimes tiny bundle scars can be seen. These scars are found in the leaf scar and mark the location of food and water conducting tissues. The shape and arrangement of the bundle scars can help distinguish deciduous trees in the winter months when the leaf structures are absent.
The origin and development of stems
Recall that the apical meristem is responsible for vertical growth, or increase in length of a stem. Prior to the start of the growing season, the cells in the apical meristem are dormant. The apical meristem is protected at the tip of the twig, by the covering bud scales and by the leaf primordial. The leaf primordia are tiny embryonic leaves that will develop into mature leaves after bud scales drop off and growth commences. When a seed germinates or a bud begins to grow, the cells in the apical meristem undergo mitosis. From these cells three primary meristems will develop:
1. The outermost meristem is the protoderm, which gives rise to the epidermis. This layer is usually one cell thick and becomes coated with a waxy cuticle.
2. The second layer is the procambium, which is a cylinder of strands. This layer gives rise to the primary xylem and primary phloem cells.
3. The innermost meristem is the ground meristem from which arises two tissues composed of parenchyma cells. The tissue in the center of the stem is the pith. These cells are large and may break down shortly after being formed which leaves a cylindrical hollow area. If they do not break down, they will be compressed by new additions to the plant girth by other meristems. The second parenchyma tissue that arises is called cortex. Cortex may be quite extensive and also crushed or replaced in woody stems. The function of both tissues is food storage. If chloroplasts are present the tissues may function in producing food.
It is important to note that all five of the above mentioned tissues—epidermis, primary xylem, primary phloem, pith and cortex—are produced by the apical meristem and are thus primary tissues as the plant is increasing in length. Xylem and phloem tissue branch off from the main vascular cylinder and enter into the leaf or bud. Each branching of vascular tissue is called a trace. Each trace branch leaves a small thumbnail shaped gap in the cylinder of tissue and are called leaf gaps and bud gaps.
In between the primary xylem and primary phloem a thin band of cells retains its meristematic nature. This band becomes the vascular cambium of one of the two lateral meristems.
In woody plants, and some others, a second cambium arises from the cortex or sometimes the epidermis or phloem. The second cambium is called the cork cambium or phellogen and is responsible for producing cork cells. Recall that the cork cells become filled with suberin which waterproofs the cells. The resulting cork tissue constitutes the out bark of woody plants and functions to reduce water loss and to protect the stem against mechanical injury. We will revisit the role of cork later on in discussing biotechnology and propagation. For now, though, understand that cork tissue cuts off food and water supplies to the epidermis which results in a sloughing off. Also understand that cork tissues do not form a solid cylinder around the exterior of a woody stem. This is to allow vital gas exchange with the environment.
Before we go on, it is important to remember the difference between monocots and dicots, the two main divisions of flowering plants. Most of the distinguishing revolves around the seed leaves, which are called cotyledons. Cotyledons function in storing food needed by the young seedling until true leaves grow and are able to take over the food supplying function.
1. Monocotyledon (monocot) plants—These plants form from seeds that have one embryonic seed leaf (hence the ‘mono’ in monocot).
2. Dicotyledon (dicot) plants—These plants form from seeds that have two (hence the ‘di’ in dicot) embryonic seed leaf.
Cone bearing trees, conifers such as pines, have multiple cotyledons, usually eight, in their seed structure.
There are four tissue patterns to be aware of in the study of plants.
1. Steles—steles are a central cylinder in most younger stems and roots, composed of primary xylem, phloem and the pith, if present. Sometimes referred to as eusteles, which are vascular bundles in higher vascular plants.
2. Herbaceous Dicotyledonous Stems—Herbaceous refers to non-woody plants. Plants that die after going from seed to maturity are called annuals. In general, most monocots are annuals, but there are annual dicot plants as well. Annual dicots are mostly composed of primary tissues, although there may be some minimal secondary growth. Remember, the plant only lives a year, so extensive secondary growth, or increases in width, really doesn’t make sense as far as using the plant resources. A cross-section of a herbaceous dicot stem will show discrete patches of xylem and phloem, vascular bundles, that are arranged in a proper ring separating the cortex and the pith. If secondary xylem and phloem are to emerge, they will arise from between the two primary tissues. Monocots will be discussed shortly.
3. Woody Dicotyledonous Stems—Wood is essentially secondary xylem growth. These stems look similar to herbaceous dicot stems up until the vascular cambium and the cork cambium start functioning. The differences are then quite obvious. While some tropical trees demonstrate year round secondary growth, most trees in temperate climates grow in the spring and summer and cease through the winter. In the springtime, when water and resources are plentiful, the vascular cambium produces large xylem cells. During the summer months when resources and water may be lacking or reduced, the xylem cells are small. Pressed up against the large, light colored xylem cells, the small xylem cells look like a thin dark ring. One year of xylem growth, called an annual ring, can be measured as the distance between the dark rings—or the distance between summer xylem growth. Summer growth is called summer wood while the large spring cells are called spring wood. Much can be learned about the local environmental conditions through the years by looking at tree rings. If water is plentiful the rings will be wider than usual. Years with fires and blights will be evident, as well as insect infestations and fungal infections. All this by looking at a cross section of a tree. In conifers, vessels and fibers are absent and thus the wood consists mainly of tracheids. It is important therefore to remember that environmental conditions affect xylem production and the dark rings may not be completely visible, one year’s growth is what constitute an annual ring, not just dark circles.
The vascular cambium produces more xylem than phloem. In fact, the phloem will be difficult to locate as the cells are thinner than xylem and more likely to collapse under the pressure of the cambiums. Phloem grows to the outside of the vascular cambium and xylem grows to the inside. The oldest xylem is in the very center of the stem/trunk. The wood in the center is called heartwood. It is usually darker as the vessels and tracheids are filled with old resins, gums and tannins. The younger wood where the xylem is still functioning is toward the outside of the stem nearest the cambium and is lighter in color. This younger wood is called sapwood. The main role of heartwood is structure and support, since it is unable to conduct water and nutrients. The heartwood sometimes rots out of a otherwise living tree. Sapwood develops at roughly the same rate that heartwood is ‘retiring’ and thus vital conducting functions are not compromised. Recall that conifers do not have vessels or fibers and are primarily tracheids. Conifers have resin canals scattered throughout the xylem tissue. Conifers are primarily considered to be softwoods while the wood of woody dicot trees are considered to be hardwood.
Bark is all of the tissues outside of the cambium, including the phloem. Some have gone so far as to distinguish between inner bark—primary and secondary phlolem and outer bark—the periderm, which consists of cork tissue and cork cambium. The cells in these layers only function briefly as they usually become crushed and then slough off. New layers are annually produced by the cambiums. The youngest phloem cells are the ones nearest the vascular cambium and are most active in transporting nutrients, sugars and water. Mature bark may be composed of alternating layers of crushed phloem and cork.
1. Monocotyledonous Stems—These plants are usually grass or grass-like and do not grow to great size. Monocot stems do not have vascular cambiums or cork cambiums, as growth will not be laterally. The vascular bundles produced by the procambium are scattered throughout the stem, rather than organized in rings as in woody dicot stems. Every bundle is oriented with the xylem toward the center of the stem and the phloem toward the stem surface. The xylem in the vascular bundle generally consists of two large vessels with some small vessels in between them, while the phloem consists of sieve tubes and companion cells. The entire vascular bundle is wrapped in a sheath of sclerenchyma cells. The background tissue between vascular bundles is not divided into cortex and pith in monocots, but they do have similar function and appearance as the parenchyma cells in cortex and pith. The concentration of bundles and bands of sclerenchyma cells, give the stem the flexibility and strength to withstand the elements—such as a summer rainstorm. In grasses, there is an intercalary meristem at the base o each internode which contributes to growth in length, like apical meristems. During the growing season, the stems of the grasses elongate rapidly. Because there is no vascular cambium that would produce tissues to increase the girth of the plant, the growth is columnar with very little variation in diameter between the top and the bottom of the plant.
Palm trees are special, because they grow to considerable size, however this is primarily due to the subsequent division and growth of their parenchyma cells. All this growth occurs without a true cambium developing. Other monocot stems have adaptations that allow for specialized growth. Monocot fibers, such as manila hemp and sisal, come from stems and leaves and are used for commercial products however, their fibers are not as strong as dicot fibers.
Rhizomes—horizontal stems that grow beneath the ground, but near the surface
of the soil. They resemble roots, but are actually modified stems with
scale-like leaves and buds at each axillary node. In addition, adventitious roots are produced along the rhizome on the lower surface in order to increase absorption surface area.
Runners and Stolons—Runners are horizontal stems that grow above ground,
usually along the surface (compare with rhizomes). Strawberry plants produce runners after the first flowering of the season, they may extend out up to 3 feet or more beyond the parent plant. Along the runner, adventitious buds will develop in order to propagate new plants. Stolons are similar to runners, except that they grow roughly vertically beneath the surface of the soil. Irish potato plants have tubers at the tips of stolons.
Tubers—Tubers develop at the tips of stolons. The plant accumulates food at the
stolon and the area swells at the internodes. When the tuber is mature the stolon will die and the ‘eyes’ of the potato are actually nodes arranged in a spiral around the modified stem. Each eye has an axillary bud in the axil of a tiny leaf, which is not always visible in maturing tubers.
Bulbs—These are actually large buds with a small stem at the lower end that is
surrounded with fleshy leaves. Onions, irises and tulips are good examples of bulbs and their main function is food storage.
Corms—On first glance you might think these guys are bulbs, however, the
differences lie beneath the thin layer of leaves covering the outside of the corm. Adventitious roots form beneath the fullness of the base. Corms function in storing food. Crocuses and gladioli are good examples of plants with corms.
Cladophylls—These are usually called the prickly part of a cactus. Cladophylls
are flattened and somewhat leaf-like in appearance. They center of each cladophyll usually has a node with small scalelike leaves complete with axillary buds. The scaly look to asparagus are cladophylls. These specialized stems are not only restricted to cacti, but are found in some orchids and greenbriars.
Other Specialized Stems—Cacti usually have modifications in their stem or
‘trunk’ structure in order to hold extra water. Other stems may be modified into thorns or briars. It is important to remember that not all thorn-like structures are stems! Raspberry and rose prickles are extensions of their epidermis and are neither thorns nor spines. Other stems are modified for climbing, such as tendrils and ramblers.
Stems are vital to the human cause. They provide building materials, paper products, food and much, much more! Stop and think of how
Leaves are covered with a thin layer of epidermal cells which permit light to the interior of the leaf, yet protect the cells from physical damage. In addition to photosynthesis leaves are involved in other vital plant functions. Respiration is a metabolic process which produces waste products. These products are deposited outside the plant when the leaves are shed. Leaves are also important to the movement of water absorbed by the roots and transported throughout the plant. The water that reaches the leaves mostly evaporates off into the atmosphere via transpiration. Leaves are complex plant organs upon which life depends. We will look into all of these processes in more detail and see just how vital leaves are to sustaining plant and animal life.
Leaf Arrangements and Types
There are over 275,000 different kinds of plants and most of them can be distinguished from each other by their leaves alone. As mentioned in the last tutorial, leaves originate as primordial in the buds regardless of their ultimate size and shape. When all is said and done, leaves usually consist of a stalk, the petiole and a flattened blade, the lamina, which has a network of veins also known as the vascular bundles. Some leaves have a pair of appendages called stipules at the base of their petiole. In some cases, there is no petiole or stalk, and these leaves are called sessile. Deciduous trees generally lose their leaves once a year, after the growing season. Evergreens, or conifers, usually are only functional for two to seven years.
The overall arrangement of leaves with respect to the plant stem is called phyllotaxy. Leaves may be arranged in an alternate, opposite pattern if they are attached at the same node, or a whorled pattern if three or more are attached at a node. The leaf itself may be a simple leaf, which has an undivided blade; or a compound leaf, in which the blade is divided into leaflets in various ways. A pinnately compound leaf has leaflets in pairs along a central stalk—called the rachis. A palmately compound leaf has all its leaflets attached at the same point on the end of the petiole. The leaflets of a pinnatley compound leaf are sometimes subdivided into even smaller leaflets which makes a bipinnately compound leaf. The venation, or arrangement of vascular bundles, in a leaf blade or a leaflet may be either pinnate or palmate. A pinnately veined leaf has a main vein called the midrib with secondary veins branching out from it. However, in a palmately veined leaf, several veins branch out from the base of the blade—rather than from a central midrib. Monocot plants generally have leaves with parallel venation as compared with dicots, which have branching and diverging veins. The Ginkgo tree is special in that it has no midrib or other large veins. The veins fork evenly and progressively from the base of the blade out to the opposite margin of the leaf. This arrangement is called dichotomous (branching) venation.
In cross section there are three major regions to see in the inside of a leaf: epidermis, mesophyll and veins—or vascular bundles. The epidermal layer is one cell thick and covers the entire surface of the leaf. On the lower surface of the leaf blade, the epidermis is interrupted by stomata. Which will be discussed shortly. From the top, the epidermal cells look like jigsaw puzzle pieces fit tightly together. The guard cells in the lower epidermal layer contain chloroplasts, but otherwise the epidermal cells do not have any chloroplasts and function as primary protection for the cells beneath. Most leaves have a thin covering of waxy cuticle.
Stomata
Stomata distinguish the lower epidermis from the upper epidermis. The upper epidermis is generally uninterrupted, but the lower epidermis is perforated by numerous tiny pores called stomata. The stomata (stoma singular) are very numerous and facilitate gas exchange between the interior of the leaf and the environment. Each stoma is regulated by a pair of sausage-shaped guard cells. They, as mentioned earlier, are the only cells in the epidermis with chloroplasts for photosynthesis. The photosynthetic products in the guard cells provide the energy for the functioning of the cells. The walls of the guard cells are thickened, except for the side adjacent to the pore. The cells will expand or contract with changes in the amount of water in the cells, hence the need for energy as the water is moved into and out of the guard cells. When the guard cells are full of water the stoma pore is open and when the water is evacuated the pore is closed.
Mesophyll and Veins
The majority of photosynthesis takes place in the mesophyll between the upper and lower epidermis layers. Usually the two layers of mesophyll can be distinguished from each other. The upper region is made of cells that look like short posts in two rows. These cells are parenchyma cells and make up the palisade mesophyll tissue. It is this tissue that contains more than 80% of the chloroplasts in the leaf. The lower layer of mesophyll, the spongy mesophyll tissue, is composed of loosely arranged parenchyma cells with abundant air space. The lower layer also contains many chloroplasts and its loose structure allows for movement of air in from the stomata. For future reference, parenchyma tissue containing numerous chloroplasts is called chlorenchyma tissue. It is also found in the outer parts of cortex in the stems of herbaceous plants. However, in the leaf, the surfaces of the mesophyll that come into contact with the air are moist. The stomata will close if the internal moisture drops below a certain level in order to reduce drying inside the leaf.
The skeleton of a leaf are the veins, or vascular bundles. They are of various sizes and as described in the leaf arrangement section, are scattered throughout the leaf and are organized distinctly in different types of leaves. The veins are surrounded by a jacket of fibers called the bundle sheath. The sugars produced in the mesophyll are transported via the veins throughout the plant—specifically in solution in the phloem. In dicots, the veins run in all directions. In monocots, the veins are parallel and are not scattered. In addition, monocots do not have mesophyll differentiated into two layers. Instead, some will have large thin-walled buliform cells surrounding the main vein. The thin-walled cells are sensitive to water conditions and will collapse in dry conditions which causes the leaf blade to fold or roll which reduces transpiration (water loss).
Depending on the conditions where a particular plant lives, it may or may not require some specialized adaptations in order to accommodate various environmental factors: humidity, temperature, light, water, and soil conditions for example. We will look briefly at ten types of specialized leaves. I would suggest further research if you are interested in more detail.
1. Shade Leaves—In some plants, leaves with barely noticeable or unnoticeable modifications will occur right alongside those that are unmodified. Leaves in the shade tens to be thinner and have fewer hairs than those on the same tree exposed to direct light. In addition, they are generally larger and have less defined mesophyll layers and reduced numbers of chloroplasts than their better lit counterparts.
2. Leaves of Arid Regions—In growing environments with extremely arid conditions, the plants will generally have thicker more leathery leaves. Their stomata are usually reduced iumber and are sunken into the leaf surface in special depressions. Some may have succulent leaves or no leaves at all—where the stem takes over photosynthetic responsibilities—or they may have dense hairy coverings. In areas where the soil freezes and water resources are limited, pine trees may have modifications similar to desert plants. Including sunken stomata, thicker cuticle and a hypodermis (thick walled cells) beneath the epidermis. The compass plant is a unique example of growth set up directionally—East and West—in order to reduce moisture loss.
3. Tendrils—Many plants have modified leaf structures called tendrils that aid in climbing or supporting the plant’s weight. Tendrils are very sensitive to contact and can be readily redirected based on touch and solid contact. Tendrils become coiled like springs and when contact with a support structure is made, the tip not only coils around it but the tip direction reverses. It needs to be noted that not all tendrils are modified leaves, tendrils of the grapevine, for example, are modified extensions of the stem tissue.
4. Spines, Thorns and Prickles—Desert plants have leaves modified as spines. Water loss is correlated to surface area, so the decrease in leaf surface area consequently decreases water loss to the outside. In plants with spines, photosynthesis is generally conducted by the stem tissue. The tissue is made of sclerenchyma cells and replaces any ‘normal’ leaf tissues. The modifications arising in the axils of leaves are stem modifications not leaf spines, but thorns. Recall, that the prickles of roses and raspberries are not leaves or stems, but outgrowths of the epidermal or cortex just beneath the prickle.
5. Storage Leaves—Succulent leaves are leaves modified to retain and store water. Water storage is permitted because of the thin-walled, non-chloroplast parenchyma cells just beneath the epidermis and to the interior of the chlorenchyma tissue. The vacuoles in the non-photosynthetic cells store the extra water resources. There are plants with succulent leaves that have a special photosynthetic process. We will look at these in a later tutorial. The fleshy leaves of onions and lily bulbs store large amounts of carbohydrates which are utilized by the plant in the next growing season.
6.
7. Flower Pot Leaves—the leaves of some plants, such as the Dischidia plant from tropical
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9. Window Leaves—There are at least three members of the Carpetweed family in the Kalahari desert with unique adaptations to the sandy growing environment. These plants have leaves shaped like ice cream cones. The leaves are buried in the sand, leaving the transparent dime-sized tip of the leaf exposed at the surface. The transparent surface is covered with a thick epidermis and cuticle and has virtually no stomata. This arrangement allows light nearly direct access to the mesophyll with chloroplasts inside. The plant, for the most part, is buried and away from drying winds and abrasive blowing sands. There are other examples of succulent plants with window leaves.
10. Reproductive Leaves—Walking fern leaves produce new plants at their tips. Air plants, a succulent, have little notches along their leaf margins where new plant are produced with leaves and roots of their own. The baby plants will produce even if the parent leaf is separated from the rest of the plant.
11. Floral Leaves (Bracts)—Bracts are found at the bases of flowers and are sometimes mistaken as petals. They compensate for small flowers or absent petals. The poinsettia ‘flower’ is really composed of bracts. The center cluster of tiny flowers is the main event, while the bracts do all the attracting.
12. Insect-Trapping Leaves—These plants are always attention grabbers and have intrigued folks for centuries. Plants that trap insects usually occur in swampy areas and bogs of tropical and temperate regions. Generally, the soil is lacking some vital ingredient for life and the plants utilize trapped insects and small organisms to fill the gap. The captured prizes are dissolved and absorbed by the plant. However, if insects are not available (i.e. a laboratory situation) the plants will develop if nutrients are given instead. The following four plants represent the four main mechanisms of capture.
Pitcher Plants—drowning trap
Sundews—sticky trap
Venus Flytraps—hinged trap
Bladderworts—underwater trapdoor trap
Autumn Changes in Leaf Color
As leaf cells break down after the growing season is over, the leaves tend to turn some shade of brown or tan due to a reaction between leaf proteins and tannins stored in the cell vacuoles. Prior to going completely tan or brown, the leaves usually demonstrate a wide variety of colors as they go through various stages of degeneration. In the chloroplasts of mature leaves are several groups of pigments such as green chlorophylls and carotenoids including yellow carotenes and pale yellox xanthophylls. These pigments play various roles in photosynthesis. The green chlorophylls are usually found in higher concentrations and during the season of active growth they are able to mask the other pigments. As the chlorophylls break down during the fall, the other colors become apparent. The breakdown of chlorophyll is not completely understood, however, it appears to be tied to the gradual reduction in day length. Anthocyanin, a common red pigment and betacyanin a second red color may also accumulate in the cell vacuoles as fall progresses. Anthocyanins are red if the cell sap is slightly acidic and blue if the sap is more alkali (basic). Betacyanins are restricted to several plant families, including cacti and beets. While some trees demonstrate brilliant fall displays of chlorophyll breakdown, others such as birch trees have a single shade of color in their fall leaves.
Abscission
Deciduous trees and plants, the ones who lose their leaves once a year have different cycles depending on where they are at in the world. In temperate climates, the leaves generally drop in the fall in preparation for winter and new growth comes in the spring. In tropical regions, the cycle follows the cycles of wet and dry seasons. Evergreen trees do shed their leaves, however not all at once or even annually. Abscission is the process in which leaves shed; whether deciduous or evergreen.
At the base of the petiole, stalk, of each leaf there is an abscission zone. Changes that take place in this region ultimately result in the drop of leaves. Hormonal changes take place as the leaf ages and two layers of cells become differentiated. (In young leaves hormones prevent these cells from differentiating.) The cell layer closest to the stem becomes the protective layer which is usually several cells deep and suberized, or coated with a fatty suberin substance. The other layer, the separation layer, forms on the leaf side of things. The cells swell and become like jelly. The pectins in the middle lamella of the cells in the separation layer are broken down by enzymes until an external event causes the leaf to fall: this could include the force of gravity overcoming the strength of the strands of xylem holding the leaf to the petiole, thus breaking it off at the gelatinous zone, wind, rain, animals etc. The pectin breakdown begins in response to environmental conditions such as dropping temperature, lack of adequate water, decreasing day lengths, changing light intensities, or damage to the leaf.
Importance to humans
Leaves are vital to humans. Not just for food but many medicines come from plant leaves. Tobacco products come from leaves, as do some hemp products and other textile fibers. Cocaine and aspirin are from leaves as are some insecticides. Aloe vera for the relief of burns—even x-ray burns will respond to aloe vera. Leaves are also used in floral arrangements and other products of aesthetic value. Bottom line: leaves, like stems are of great value to humans.
Literature
1. Botany / Randy Moore, W.Denis Clark, Kingsley R.Stern, Darrell Vodopich. – Dubuque, IA, Bogota, Boston, Buenos Aires, Caracas,Chicago, Guilford, CT, London, Madrid, Mexico City, Sydney, Toronto: Wm.C.Brown Publishers.- 1994.-
2. Kindsley R. Stern. Introductory plant biology-
3. Gulko R.M. Explanatory Dictionary of Medicinal Botany- Lviv: LSMU, 2003.-200 p.
4. Raven, P. H., R. F. Evert, & S. E. Eichhorn. Biology of Plants, 7th ed., page 9. (
5. Harold C. Bold, C. J. Alexopoulos, and T. Delevoryas. Morphology of Plants and Fungi, 5th ed., page 3. (New York: Harper-Collins, 1987). ISBN 0-06-040838-1.
6. Winterborne J, 2005. Hydroponics – Indoor Horticulture [1]
7. Leopold, A. C. Plant Growth and Development, page 183. (New York: McGraw-Hill, 1964).
Prepared by ass. prof. Shanayda M.I.
