Theme 4
MORPHOLOGY OF VEGETATIVE ORGANS OF PLANTS
Vegetative organs are widely used in medicine. For
visual identification of plants it is necessary to know their morphological features. That is why, learning of morphology of different types of roots, rhizomes, stems, and leaves and their modifications are necessary for future pharmacists.
Plant morphology observes the vegetative (somatic) and reproductive structures of plants. The vegetative (somatic) structures of vascular plants include two major organ systems: a shoot system, composed of stems and leaves, and a root system. The reproductive structures are more varied, and are usually specific to a particular group of plants, such as flowers and seeds, fern sori, and moss capsules. 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. This area of plant morphology overlaps with the study of biodiversity and plant systematics.
When characters are used in descriptions or for identification they are called diagnostic or key characters which can be either qualitative and quantitative. Both kinds of characters can be very useful for the identification of plants.
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.
ROOT
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 brith (that is, growing up above the ground or especially above water). 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,
2) anchoring the plant body to the ground.
They often function in storage of food (carrot). 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.
Types of roots
A root system consists of a primary root, secondary roots (or lateral roots) and additional roots (growth from stem, leaf).
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:
I. The taproot ystem: 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 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.
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.
SHOOT
SHOOT is the vegetative organ of plants, that include the stem, leaf and bud, also node and internode.
Stem – an axial organ of plants, the part of shoot
Bud – a compressed, undeveloped shoot. Buds may be lateral or terminal.
Node – point on the stem where leaf or bud is borne. The space between two nodes is an internode
Petiole–the stalk of a leaf; a leaf without a petiole is sessile
Blade–the flat, expanded portion of the leaf
STEM is an axial organ of plants, the part of shoot. Function of stem:
1) Stem transport water and solutes between roots and leaves
2) Stem support leaves
3) Stems store materials
4) Yung green stems are photosyntetate.
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 according to position in the space may be straight (wheat), creeping (strawberries), recumbent, climbing (convolvulus – hop-plant), clinging (tenacious – ivy)
The shape of stem in cross-section: cylindrical, triangular, elliptic,
square, oval, etc.
LEAF
Leaves are the most active and conspicous organs of plants. The most impotant of their function is absorbing sunlight for photosynthesis.
Leaves are the most diverse of all plant organs – they can be tubular (onion), needlike (pine), elliptic and other, simple and compaund. Stems are often specialized for storage, asexual reproduction, protection
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.
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
|
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)
COMPOUND LEAVES – the blade is divided all the way to the midrib (rachis) into two or more pieces.
Pinnately compound
– leaflets arranged along one undivided main axis (rose, rowan-tree).
Palmately compound–leaflets all arising from one point at the base of the leaf (horse chestnut)
|
Three-leaves compaund (ternate) – (clover, strawberry )
- 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).
|
Venation (arrangement of the veins)
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).
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).
|
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.
Fall of the leave
Evergreen-having leaves which persist for two or more seasons. Broadleaf evergreens usually have thick, leathery leaves.
Deciduous–having leaves which die and fall in the cold or the dry season every year.
References
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.
Prepared by ass. –prof. Shanayda M.I.
