Basic Plant Tissues

3.1.10 Microtubules

Microtubules are hollow, long, straight structures composed of protein sub-units and are located in the periphery of the cytoplasm, closest to the cell wall. They are also found in the mitotic and meiotic spindles and in another cell division-related structure, the phragmoplast, which appears in telophase.

Since microtubules are associated with cell division and near cell walls where growth is still occurring, one theory is that microtubules provide direction for developing microfibrils.

3.1.11 Mitochondria

Mitochondria are involved in cellular respiration and can be seen under a light microscope with the Janus Green B stain. They can look lobed, elongated, or spherical when cells are sectioned. These organelles have two unit-membranes:

the internal membrane forms many folds into the matrix (primarily composed of protein), and the outer membrane surrounds the inner membranes. The mitochondria are involved in energy conversion and contain enzymes used for the Krebs cycle.

3.2 Basic Plant Tissues

3.2.1 Meristem

A plant embryo, in the early stages of development, will undergo continuous cell division until the adult plant body is formed. After plant cell differenti-ation into specialized tissues has occurred, embryonic-like cell division is restricted to certain regions of the plant called meristems (see Figure 3.2).

The definition of meristematic tissue includes cells that have the ability to continue on with unlimited numbers of cell division. Other young tissues

Figure 3.2 The meristematic region of a plant is defined as an area of actively dividing cells that can give rise to new organs and tissues. Apical meristems are located at the tips of shoots and roots; lateral meristems are cylinders of cells found on the sides of roots and shoots.

peripheral meristem

leaf primordium quiescent zone

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may continue to exhibit cell division, but those numbers are limited. Meristems are classified into different types based on the following:

• Position in plant body

• Their structure

• Their function

• Their stage of development

• Their origin

• The types of tissues they produce

Based on their position in the plant body, meristems can be apical (located in the apices of main and lateral shoots and roots), intercalary (located between mature tissues), or lateral (positioned parallel to the circumference of the organ where they are located). Meristems are often referred to as primary (derived from embryonic cells) or secondary (derived from mature differentiated tissues) based on the tissues they originate from. Meristems are responsible for generating new roots, shoots, and flowers on the body of the plant as well as extending the length of parts of a plant body, such as the stem.

The cellular patterns of division within these different types of meristems have been mapped and studied extensively in order for developmental biologists to better understand the ordered plan for generating a plant body and specific plant organs.

3.2.2 Parenchyma

Parenchyma is comprised of living cells that may differ in shape and function.

These cells retain their ability to divide even when mature and thus are able to assist in wound recovery and tissue regeneration. Parenchyma cells are considered somewhat primitive because many of the phylogenetically lower plants consist only of parenchyma cells. The following parts of a plant contain parenchyma cells:

• Pith (the ground tissue in the center of the stem and root)

• Root cortex (tissue between the vascular cylinder and root epidermis)

• Shoot cortex (tissue between the vascular cylinder and shoot epidermis)

• Pericycle (the ground tissue between the conducting tissues and the endodermis)

• Leaf mesophyll (the photosynthetic tissue between the two epidermis layers)

• Xylem (tissue that conducts water in vascular plants)

• Phloem (tissue that conducts nutrients in vascular plants)

• Fleshy fruits

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3.2.3 Collenchyma and Sclerenchyma

The supporting tissues of a plant are called collenchyma and sclerenchyma (see Figure 3.3). Collenchyma is made up of living, elongated cells with typ-ically uneven thickening of the cell wall. Immature collenchyma is stretchable and irreversibly expands as an organ grows and develops. Mature collenchyma is more brittle and less plastic and, like parenchyma, may contain chloroplasts and tannins. In plants exposed to shaking or heavy winds, collenchyma tissue will become increasingly thicker to aid in support of the plant body. Collen-chyma is located in the following regions of a plant body:

• Stems (in longitudinal strips or as a complete cylinder)

• Leaves (on one or both sides of a vein; along margins of a blade)

• Floral parts

• Roots

• Fruits

Sclerenchyma also functions to support the plant body and may result from lignification and wall thickening of collenchyma tissue. Unlike collen-chyma, however, schlerenchyma is elastic (which means it can expand and contract) rather than plastic (capable of expansion only). Sclerenchyma can be further subdivided into two types: fibers (long cells) and sclereids (short cells). Although size is a general trend for the classification of fibers and sclereids, it is not very accurate. A true classification is based on the origin Figure 3.3 Collenchyma and schlerenchyma are types of plant tissues that are used for support of the plant body. Collenchyma cells usually lack a secondary wall and are found in young growing tissue; schlerenchyma cells have a secondary wall with strengthening lignin and are not able to elongate in growth.

collenchyma sclerenchyma

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of each sclerenchyma type as fibers develop from meristematic cells and sclereids originate from parenchyma cells that have thickened secondary walls. Fibers may be present as single cells or as groups throughout the plant body. Most often, they comprise vascular tissues such as xylem and phloem.

Sclereids also can be found as single cells; however, most often they are present as masses of hard cells within soft parenchyma tissues. Interestingly, the shells of walnuts, for example, are composed entirely of sclereids.

3.2.4 Xylem

The vascular system of higher plants plays an important role for both physiology and phylogeny and has been used in classification of various plant species. The role of the xylem is to transport water and solutes from the roots to the arial portions of the plant body. The xylem is a complex tissue consisting of numerous cell types, including tracheary elements (tracheids and vessel members), fibers, sclereids, and parenchyma cells. The tracheary elements, in particular, are used for evolutionary phylogenetic study. The cell features of tracheary elements useful for this type of study include

• Length of the element

• Diameter of the element

• Thickness of the cell wall

• Shape of the element when viewed in cross section

• Type of pitting

• Type of perforation plate 3.2.5 Phloem

The phloem is a vascular conducting tissue for transport of photosynthetic products. The sieve elements are the cells responsible for carrying out this function. Other cell types found within phloem tissue include specialized parenchyma cells (companion cells, albuminous cells), fibers, sclereids, resin ducts, and lacticifers (cells containing latex). The position of phloem in a stem is usually external to that of the xylem tissue. However, in some plant families (e.g., Cucurbitaceae, Myrtaceae, Compositae, Solanaceae), the phloem can also be located inside the xylem. The identifying feature of a sieve element is the lack of a nucleus and a viscous substance called P-protein. P-protein stains easily with cytoplasmic stains, and with the use of an electron microscope, its features can be useful for distinguishing some plant species. P-protein appears present in all dicotyledonous species but seldom in monocots, gymnosperms, and pteridophytes. The presence of P-protein in dicot species sieve elements may be observed as many forms including filamentous, tubular, granular, and crystalline.

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3.2.6 Secretory Cells

Plant cells commonly secrete a variety of substances, including waxes, forms of latex, and resins. The term secretion, in the context of this book, will refer to the active elimination of a substance from a cell. Resin ducts are quite common in conifers. In some species, they are present as a normal state; in others, they are the result of injury to the plant. In the genera Pinus, Larix, Pseudotsuga, and Pinus, resin ducts appear as typical features of wood, and the substance they produce is commonly called “pitch.” Plant “gums” are the result of a process termed gummosis, which is the metamorphosis of primary and secondary cell wall matter into disorganized substances called gums. As the cell walls break down, the cavity is filled with starches and gums.

Gummosis occurs often as a result of mechanical injury to the plant through the action of insects or diseases, or by physiological disturbance. Some gums are commercially important and can be distinguished from one another using capillary electrophoresis and polarized light microscopy. These types of chemical analyses are useful when determining purity of a compound, which may be important to food items or manufacturing processes. Latex occurs as a suspension or emulsion and is common to many angiosperms. The chemical composition and color of latex differs in various plant species and may be used as a classification feature. Within the latex, salts, organic acids, and other substances may occur as in the following:

• Compositae (sugars)

• Musa (tannins)

• Papaver somniferum (alkaloids)

• Carica papaya (enzymatic papain)

Perhaps the best example of a secretory plant, and one with worldwide economic importance, is the Para rubber tree (Hevea brasiliensis). The latex of H. brasiliensis contains approximately 30% rubber, and it is grown in Central America, the West Indies, Brazil, Liberia, Sumatra, Ceylon, Java, eastern India, and the Malayan Archipelago. During World War II, when rubber was inaccessible from Asia, other rubber-containing plants were identified as secondary economic sources of rubber:

• Castilla (Panama rubber)

• Manihot (Ceara rubber)

• Parthenium argentatum

• Taraxacum kok-saghyz

• Hancornia

• Landolphia

• Cryptostegia

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Waxes are deposited commonly on the surface of the cuticle for protec-tion of plant leaves and fruits. These waxes may form a continuous layer or may be deposited in granules, rods, scales, or platelets. The wax reduces the wettability of the plant leaf surface and lends a shine to increase the appeal of certain fruits.

3.2.7 Epidermis

The epidermis is the outermost cell layer located on the exterior of leaves, stems, flowers, fruits, seeds, and roots. Both morphologically and function-ally, the epidermis is not a uniform tissue as it is comprised of numerous different cell types, some of which have discrete functions. The epidermis persists throughout organs until secondary thickening of cell walls occurs.

In some plants, the epidermis may be replaced by cork tissue as they age, especially if they have a long life span. The ordinary cells of the epidermis may vary in size and shape, but they are all tightly compacted without intercellular spaces. The epidermal cell walls can also vary in thickness, and on the outer surface, pectin may be found. Cutin, a fatty substance, stains red with the cellular stain Sudan IV and may be embedded within or along the surface of the outer walls of the epidermis. This layer of cutin forms a structure called the cuticle, which may be complex and diagnostic for certain plant species. The cuticle surface may be smooth, ridged, or furrowed. From early geological studies, recovered cuticles have retained the shape and orien-tation of the mesophyll cells below, providing important information to classify these plant forms. Within the epidermal layer are also stomata, which are intercellular spaces surrounded by specialized guard cells that control the opening and closing of these spaces. Stoma (the plural of stomata) are typically present on the arial portions of rhizomes, leaves, and stems as they play an important role in regulating transpiration rates in plants. In addition, the single or multicellular appendages of the epidermis are called trichomes (“plant hairs”). The use of stomatal spacing and trichome characteristics in phylogenetic classification is well known in forensic botany.

3.2.8 Periderm

The periderm is a secondary plant tissue consisting of three parts: phellogen (cork cambium), phellem (cork), and phelloderm (parenchyma-like tissue).

The cork functions as a protective layer to protect the plant as the epidermis dies and is shed. Cork is present in the roots and stems of many dicotyle-donous plants and, with all of the parts together, is commonly known as

“bark.” The type of bark present on trees, for instance, can be used in many cases as a taxonomic classification character (e.g., rough, smooth, and con-tinuous). Some genera have overlapping regions of periderm that produce a

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scaly bark (e.g., Pinus, Pyrus). Other genera (e.g., Vitis, Clematis, Lonicera) form continuous cylinders of periderm called “ring bark.” Still other plant species are intermediate between the scaly and ring barks (e.g., Arbutus, Platanus, and Eucalyptus).

Commercial cork is produced from many trees, in particular, Quercus suber. Cork is valued because it is impenetrable to gas and liquid and has strength and elasticity and is lightweight. Stripping the periderm from a 20-year-old tree is the first step in forming commercial cork. The exposed cells of the phelloderm and cork die, and a new phellogen is rapidly formed as a wound response. Within 10 years, this second phellogen is ready for harvest.

This process continues at 10-year intervals until the tree is approximately 150 years old. The dark brown spots that may be noted within commercial cork (e.g., wine bottle corks) are lenticels, isolated areas in the periderm consisting of suberized and nonsuberized cells.

3.3 Common Staining Techniques and Laboratory Exercises