THEME № 2
MERISTEMATIC, DERMAL, GROUND AND SECRETORY TISSUES
The tissues of a plant are organized into three tissue systems: the dermal tissue system, the ground tissue system, and the vascular tissue system.
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Tissue System |
Component Tissues |
Location of Tissue Systems |
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Dermal Tissue System |
Epidermis |
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Ground Tissue System |
Parenchyma tissue |
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Vascular Tissue System |
Xylem tissue |
Simple tissues are:
Complex tissues are:
Meristematic Tissues
Tissues where cells are constantly dividing are called meristems or meristematic tissues. These regions produce new cells. These new cells are generally small, six-sided boxlike structures with a number of tiny vacuoles and a large nucleus, by comparison. Sometimes there are no vacuoles at all. As the cells mature the vacuoles will grow to many different shapes and sizes, depending on the needs of the cell. It is possible that the vacuole may fill 95% or more of the cell’s total volume.
There are three types of meristems:
1. Apical Meristems
2. Lateral Meristems
3. Intercalary Meristems
Apical meristems are located at or near the tips of roots and shoots. As new cells form in the meristems, the roots and shoots will increase in length. This vertical growth is also known as primary growth. A good example would be the growth of a tree in height. Each apical meristem will produce embryo leaves and buds as well as three types of primary meristems: protoderm, ground meristems, and procambium. These primary meristems will produce the cells that will form the primary tissues.
Lateral meristems account for secondary growth in plants. Secondary growth is generally horizontal growth. A good example would be the growth of a tree trunk in girth. There are two types of lateral meristems to be aware of in the study of plants.
The vascular cambium, the first type of lateral meristem, is sometimes just called the cambium. The cambium is a thin, branching cylinder that, except for the tips where the apical meristems are located, runs the length of the roots and stems of most perennial plants and many herbaceous annuals. The cambium is responsible for the production of cells and tissues that increase the thickness, or girth, of the plant.
The cork cambium, the second type of lateral meristem, is much like the vascular cambium in that it is also a thin cylinder that runs the length of roots and stems. The difference is that it is only found in woody plants, as it will produce the outer bark.
Both the vascular cambium and the cork cambium, if present, will begin to produce cells and tissues only after the primary tissues produced by the apical meristems have begun to mature.
Intercalary meristems are found in grasses and related plants that do not have a vascular cambium or a cork cambium, as they do not increase in girth. These plants do have apical meristems and in areas of leaf attachment, called nodes, they have the third type of meristematic tissue. This meristem will also actively produce new cells and is responsibly for increases in length. The intercalary meristem is responsible for the regrowth of cut grass.
There are other tissues in plants that do not actively produce new cells. These tissues are called nonmeristematic tissues. Nonmeristematic tissues are made of cells that are produced by the meristems and are formed to various shapes and sizes depending on their intended function in the plant. Sometimes the tissues are composed of the same type of cells throughout, or sometimes they are mixed. There are simple tissues and complex tissues to consider, but we will start with the simple tissues for the sake of discussion.
Primary Meristems
A primary meristem arises in the tissue of the embryo and continues to exist in the plant organ in which it rose. The primary meristem found at the tips of stems or roots is called the apical meristem which is responsible for increase in length as it gives rise to the first or primary permanent tissues. Apical meristems are composed of the following:
- The promeristem: this is one cell, or a number of cells, which, by cell division (mitosis), gives rise to the histogens or regions of active cell division and tissue formation.
- The histogens: these arise from the promeristem and divide repeatedly to form the primary permanent tissue. Different histogens, which give rise to different permanent tissues, can be distinguished, namely:
- the protodermis from which the epidermis differentiates,
- the ground meristem which give rise to the cortex,
- the procambium from which the pericycle and first vascular bundles develop,
- the calyptrogen (which is found only in roots) from which the root cap (calyptra) differentiates.
Secondary Meristems
In flowering plants, meristems develop from cells that suspend their ability to divide, and resume this activity later. Such meristems are known as secondary meristems. These cells give rise to permanent secondary tissues.
Covering tissues are:
Epidermis, epiblema, periderm.
Epidermis
The epidermis is also a complex plant tissue, and an interesting one at that. Officially, the epidermis is the outermost layer of cells on all plant organs (roots, stems, leaves). The epidermis is in direct contact with the environment and therefore is subject to environmental conditions and constraints. Generally, the epidermis is one cell layer thick, however there are exceptions such as tropical plants where the layer may be several cells thick and thus acts as a sponge. Cutin, a fatty substance secreted by most epidermal cells, forms a waxy protective layer called the cuticle. The thickness of the cuticle is one of the main determiners of how much water is lost by evaporation. Additionally, at no extra charge, the cuticle provides some resistance to bacteria and other disease organisms. Some plants, such as the wax palm, produce enough cuticle to have commercial value: carnauba wax. Other wax products are used as polishes, candles and even phonographic records. Epidermal cells are important for increasing absorptive surface area in root hairs. Root hairs are essentially tubular extensions of the main root body composed entirely of epidermal cells. Leaves are not left out. They have many small pores called stomata that are surrounded by pairs of specialized epidermal cells called guard cells. Guard cells are unique epidermal cells because they are of a different shape and contain chloroplasts. They will be discussed in detail later on in the tutorial. There are other modified epidermal cells that may be glands or hairs that repel insects or reduce water loss.
Functions:
- the epidermal cells protect the underlying cells,
- the waxy cuticle prevents the loss of moisture from the leaves and stems,
- the transparent epidermal cells allow sunlight (for photosynthesis) to pass through to the chloroplasts in the mesophyll tissue,
- the stomata of leaves and stems allow gaseous exchange to take place which is necessary for photosynthesis and respiration,
- water vapour may be given off through the stomata during transpiration,
- the root-hairs absorb water and dissolved ions from the soil.
Guard Cells
To facilitate gas exchange between the inner parts of leaves, stems, and fruits, plants have a series of openings known as stomata (singular stoma). Obviously these openings would allow gas exchange, but at a cost of water loss. Guard cells are bean-shaped cells covering the stomata opening. They regulate exchange of water vapor, oxygen and carbon dioxide through the stoma.
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A diagrammatic representation of a stomata, |
In botany, a stoma (also stomate; plural stomata) is a pore, found in the leaf and stem epidermis that is used for gas exchange. The pore is formed by a pair of specialized parenchyma cells known as guard cells which are responsible for regulating the size of the opening. Air containing carbon dioxide enters the plant through these openings where it is used in photosynthesis and respiration. Oxygen produced by photosynthesis in the spongy layer cells (parenchyma cells with pectin) of the leaf interior exits through these same openings. Also, water vapor is released into the atmosphere through these pores in a process called transpiration.
Stomata are present in the sporophyte generation of all land plant groups except liverworts. Dicotyledons usually have more stomata on the lower epidermis than the upper epidermis. Monocotyledons, on the other hand, usually have the same number of stomata on the two epidermes. In plants with floating leaves, stomata may be found only on the upper epidermis; submerged leaves may lack stomata entirely.
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Carbon gain and water loss
Carbon dioxide, a key reactant in photosynthesis, is present in the atmosphere at a concentration of about 384 ppm (as of March 2008). Most plants require the stomata to be open during daytime. The problem is that the air spaces in the leaf are saturated with water vapor, which exits the leaf through the stomata (this is known as transpiration). Therefore, plants cannot gain carbon dioxide without simultaneously losing water vapor.
Evolution
The fossil record has little to say about the evolution of stomata. They may have evolved by the modification of conceptacles from plants’ alga-like ancestors.
Development
There are three major epidermal cell types which all ultimately derive from the L1 tissue layer of the shoot apical meristem, called protodermal cells: trichomes, pavement cells and guard cells, all of which are arranged in a nonrandom fashion. An asymmetrical cell division occurs in protodermal cells resulting in one large cell that is fated to become a pavement cell and a smaller cell called a meristemoid that will eventually differentiate into the guard cells that surround a stoma. This meristemoid then divides asymmetrically one to three times before differentiating into a guard mother cell. The guard mother cell then makes one symmetrical division, which forms a pair of guard cells.
Types of stomata:
• Anomocytic. No subsidiary cells. Cells surrounding guard cells are not different that other epidermal cells (Ranunculaceae family).
• Anisocytic. Guard cells surrounded by three subsidiary cells, one of which markedly smaller than the others (formerly Cluciferous- Brassicaceae).
• Paracytic. types of stomata surrounded by two subsidiary cells, oriented parallel to the stoma (formerly Rubiaceous).
• Diacytic. type the stomata with two subsidiary cells oriented perpendicular to the pore of stoma (Mint family – Lamiaceae).
• Actinocytic. Several subsidiary cells that radiate from the center of the stoma forming a ring.
Tetracytic types of stomata surrounded by four subsidiary cells (Monocot plants – lily of the valley).
STOMATAL COMPLEX TYPE HAS TAXONOMIC VALUE. MANY TAXA (GENUS, FAMILY) ARE CHARACTERIZED BY A PARTICULAR TYPE.
TRICHOMES
• Trichomes are appendages of the epidermis.
• Most are hair-like are are commonly called plant hairs.
• There is a large diversity of forms – examples:
Types of epidermal trichomes.
· Can be unicellular or multicellular
• Can be simple or branched
• May have secretory function (glandular hairs)
• Epidermal “peel” with trichomes
Pelargonium (geranium) leaf epidermis, w.m. showing stomata and two kinds of trichomes.
Trichomes occur in a multitude of forms and sizes. Although they have been used widely for taxonomic purposes, their adaptive significance has been all but ignored by the evolutionist and ecologist. It is clear that trichomes play a role in plant defense, especially with regard to phytophagous insects. Iumerous species there is a negative correlation between trichome density and insect feeding and oviposition responses, and the nutrition of larvae. Specialized hooked trichomes may impale adults or larvae as well. Trichome may also complement the chemical defense of a plant by possessing glands which exude terpenes, phenolics, alkaloids or other substances which are olfactory or gustatory repellents. In essence, glandular trichomes afford an outer line of chemical defense by advertising the presence of “noxious” compounds. In some groups of plants, protection against large mammals is achieved by the presence of stinging trichomes. Intraspecific variation for trichome type and density is known in many species, and often is clinal in accordance with ecographic parameters.
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The dermal tissue system protects the soft tissues of plants and controls interactions with the plants’ surroundings. The epidermis is a dermal tissue that is usually a single layer of cells covering the younger parts of a plant. It secretes a waxy layer called the cuticle that inhibits water loss. Some of the many types of cells in the epidermis are shown below. |
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Most epidermal cells lack chloroplasts. |
Guard cells contain chloroplasts and regulate gas exchange between the inside of the leaf and the surrounding air. |
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Epidermal hairs lower water loss by decreasing the flow of air over the plant surface, which in turn, slows the loss of water from the plant. |
Glandular hairs prevent herbivory by storing substances that are harmful to insects. |
Root hairs increase water uptake by increasing the surface area of the cell. |
Periderm
In woody plants, when the cork cambium begins to produce new tissues to increase the girth of the stem or root the epidermis is sloughed off and replaced by a periderm. The periderm is made of semi-rectangular and boxlike cork cells. This will be the outermost layer of bark. These cells are dead at maturity. However, before the cells die, the protoplasm secretes a fatty substance called suberin into the cell walls. Suberin makes the cork cells waterproof and aids in protecting tissues beneath the bark. There are parts of the cork cambium that produce pockets of loosely packed cork cells. These cork cells do not have suberin imbedded in their cell walls. These loose areas are extended through the surface of the periderm and are called lenticels. Lenticels function in gas exchange between the air and the stem interior. At the bottom of the deep fissures in tree bark are the lenticels.
The outer layer of periderm, cork tissue, is composed of dead cells whose cell walls are impregnated with a waxy material, suberin.
Parenchyma
Parenchyma is the most common plant tissue. It is relatively unspecialized and makes up a substantial part of the volume of a herbaceous plant and of the leaves, flowers and the fruits of woody plants. The thin-walled parenchyma cells have large vacuoles and distinct intercellular spaces.
Functions:
- the most important function of the parenchyma cells of roots and stem is the storage of food (e.g. starch) and water,
- the intercellular air spaces permit gaseous exchange.
Chlorenchyma
Chlorenchyma cells are actually parenchyma cells, but they contain chloroplasts, e.g. the parenchyma cells of leaves and stems. The mesophyll cells of leaves can thus be regarded as chlorenchyma.
Functions:
- the chlorenchyma are the main photosynthetic cells of the plant and manufacture carbohydrates during photosynthesis.
Secretory Cells and Tissues
As a result of cellular processes, substances that are left to accumulate within the cell can sometimes damage the protoplasm. Thus it is essential that these materials are either isolated from the protoplasm in which they originate, or be moved outside the plant body. Although most of these substances are waste products, some substances are vital to normal plant functions. Examples: oils in citrus, pine resin, latex, opium, nectar, perfumes and plant hormones. Generally, secretory cells are derived from parenchyma cells and may function on their own or as a tissue. They sometimes have great commercial value.
SECRETORY TISSUES
Plants secrete a variety of substances from structures сalled secretory structures.
There are two types of secretory structures:
– external secretory structures (nectaries, hydathodes, secretory hair)
– internal secretory structures (secretory cells, canals, ducts, cavities, laticifers).
EXTERNAL SECRETORY STRUCTURES
NECTARIES are structures that secrete nectar. Most nectaries are associated with flowers and are called floral nectaries. Their nectar is 10-50 % sugar and also contains amino acids.
HYDATHODES are tissues that secrete water via a process called guttation.
SECRETORY HAIR of many plants are secretory or grandular. Such hairs commonly have a head composed of one or more secretory cells.
INTERNAL SECRETORY STRUCTURES
SECRETORY CELLS are large cells containing substances such as oil, tannins, resins and other. They often occur in groups and have several functions, including the storage and production of chemical deterrents to foraging animals.
CANALS, DUCTS, CAVITIES . Many plants secrete oil and resins into internal canals, ducts and cavities. For example, Citrus oil, Eucalyptus oil, and pine (Pinus) are extracted from internal cavities, ducts and canals, these oils deter glazing animals.
LATICIFERS is a type of elongated secretory cell found in the leaves and/or stems of plants that produce latex (contain a ordanic acids, alkaloids, oil, resins) as secondary metabolites. Laticifers may be articulated, i.e., composed of a series of cells joined together, or non-articulated, consisting of one long cell. Laticifers occur in several families of plants.
Cells or organizations of cells which produce a variety of secretions. The secreted substance may remain deposited within the secretory cell itself or may be excreted, that is, released from the cell. Substances may be excreted to the surface of the plant or into intercellular cavities or canals. Some of the many substances contained in the secretions are not further utilized by the plant (resins, rubber, tannins, and various crystals), while others take part in the functions of the plant (enzymes and hormones). Secretory structures range from single cells scattered among other kinds of cells to complex structures involving many cells; the latter are often called glands.
Epidermal hairs of many plants are secretory or glandular. Such hairs commonly have a head composed of one or more secretory cells borne on a stalk. The hair of a stinging needle is bulbous below and extends into a long, fine process above. If one touches the hair, its tip breaks off, the sharp edge penetrates the skin, and the poisonous secretion is released.
Glands secreting a sugary liquid the nectar in flowers pollinated by insects are called nectaries. Nectaries may occur on the floral stalk or on any floral organ: sepal, petal, stamen, or ovary.
The hydathode structures discharge water—a phenomenon called guttation—through openings in margins or tips of leaves. The water flows through the xylem to its endings in the leaf and then through the intercellular spaces of the hydathode tissue toward the openings in the epidermis. Strictly speaking, such hydathodes are not glands because they are passive with regard to the flow of water.
Some carnivorous plants have glands that produce secretions capable of digesting insects and small animals. These glands occur on leaf parts modified as insect-trapping structures. In the sundews (Drosera) the traps bear stalked glands, called tentacles. When an insect lights on the leaf, the tentacles bend down and cover the victim with a mucilaginous secretion, the enzymes of which digest the insect. See also Insectivorous plants; Venus’ flytrap.
Resin ducts are canals lined with secretory cells that release resins into the canal. Resin ducts are common in gymnosperms and occur in various tissues of roots, stems, leaves, and reproductive structures.
Gum ducts are similar to resin ducts and may contain resins, oils, and gums. Usually, the term gum duct is used with reference to the dicotyledons, although gum ducts also may occur in the gymnosperms.
Oil ducts are intercellular canals whose secretory cells produce oils or similar substances. Such ducts may be seen, for example, in various parts of the plant of the carrot family (Umbelliferae).
Laticifers are cells or systems of cells containing latex, a milky or clear, colored or colorless liquid. Latex occurs under pressure and exudes from the plant when the latter is cut.
