Anatomy of Plants

Cells

As in all other organisms, cells make up the fundamental unit of the plant body. Plant cells, whether formed in vegetative or reproductive organs, are usually bounded by a thin, mostly cellulosic wall that encloses the living protoplast. Young cells characteristically develop numerous membrane-bound vesicles, originating from the endoplasmic reticulum (ER) and dictyosomes. Some of these immature cells retain their ability to divide and are called meristematic. As cells age, the vesicles grow together, usually forming one relatively large water-filled vacuole positioned in the cell's center, which inhibits further cell division. Some older cells additionally produce a thick lignified cell wall located between the cellulosic wall and the protoplast. Cell walls are ordinarily considered a part of the cell's nonliving environment, but considerable research has revealed that a number of important physical and chemical events take place within cell walls.

Tissues

Groups of adjacent cells having similar structure and function constitute the tissues of the plant. Tissues having a structural or functional similarity comprise the plant organs. All of the cells, tissues, and organs trace their origins back to the zygote or embryo, which develops within the seed. Meristematic tissues within the embryo and young plant are responsible for generating new cells that ultimately result in increased plant size. Apical meristems develop near the ends of all stems and roots, and they contribute to the increased length of both. Lateral meristems (cambia), such as the vascular cambium and cork cambium are responsible for increased diameter, especially noticeable in long-lived tree species. The vascular cambium is initiated more internally within the plant body, producing cells both interiorly and exteriorly. Xylem tissue is produced interiorly and phloem tissue is produced exteriorly. The thick-walled secondary xylem tissue constitutes the wood, while all tissues outside of the vascular cambium, including the cork cambium, comprise the plant's bark. Tissues produced by the apical meristems result in the primary growth of the plant, while tissues resulting from cell divisions of the cambia result in secondary growth.

One way in which plant tissues are categorized is on the basis of their cell wall thickness. Parenchyma tissue is composed of evenly thin-walled cells, having cellulosic walls. Sclerenchyma tissue is composed of evenly thick-walled cells as a result of developing both cellulosic and lignified wall layers. Collenchyma tissue is an intermediate tissue type having unevenly thickened cells aggregated together. Sclerenchyma tissue functions both in support and water conduction, while parenchyma participates in food storage, cell division, and food conduction. Collenchyma is the least common tissue type among the three and may be involved in a combination of activities associated with the other two types.

Plant tissues can also be characterized by their relative position within the plant organs. There are three tissue systems within the mature plant: the dermal, the vascular, and the ground tissue systems. Dermal tissues compose the outermost tissue layer, the epidermis. Xylem and phloem are prominent tissues comprising the vascular system of the plant. In the root, the vascular system (also referred to as the vascular cylinder or stele) has as its outer boundary a single layer of pericycle tissue that is immediately exterior to the xylem and phloem. The pith, composed of thin-walled cells, may develop immediately interior to the vascular tissues, more centrally located in stems and roots. Root pericycle is a common tissue of origin for the formation of lateral roots, while the pith tissue is involved in food storage. Positioned in between the dermal and vascular tissue systems is the ground system, often referred to as the cortex. The cortex of younger stems and roots may be further partitioned from outside to inside into the hypodermis, the cortical parenchyma, and the endodermis. The endodermis of underground roots and stems, especially, often develops waxy deposits of suberin in the cell walls. These water-impervious deposits are referred to as Casparian thickenings and are believed to prevent the diffusion of water and dissolved substances along the cell walls, directing the movement through the protoplast.

The dermal and ground tissue systems of roots and stems of older tree species may be sloughed off by the activity of the ever-increasing circumference of the vascular cambium. In these older organs another dermal tissue, the cork, is produced outwardly by the cork cambium. The enclosing cork functions not only as a protective surface layer but also as a tissue involved in water conservation.

Organs

Organs of the plant body can be classified as either vegetative or reproductive. Usually the vegetative organs are involved in procuring required nutrients for the plant. For example, the green photosynthetic leaves are responsible for the production of food, while the roots can function as excellent food storage organs, as well as obtaining water and dissolved minerals by absorption from the soil. Cellular extensions of the root epidermal cells, the root hairs, increase the absorbing surface of roots. Stem organs, developing between the leaves and the roots, produce conducting tissues (xylem and phloem) responsible for transporting food, water, and dissolved substances throughout the rest of the plant. Reproductive organs of the angiosperms are the flowers, fruits, and seeds. These three organs are ordinarily involved in sexual reproduction, while the vegetative organs may function in asexual reproduction.

Complete flowers are composed of the four appendages produced by the reproductive apical meristem of the stem: sepals, petals, stamens, and carpels. The whorl of petals can be quite conspicuous, attracting pollinators such as insects to the flower, where pollen can be inadvertently gathered from the stamens. The carpels together constitute the pistil, the component parts of which are the landing pad for the pollen (the stigma), the style, and the basal ovary. Ovules develop within the ovary, and following fertilization give rise to the seeds and fruit, respectively.

Organs of the embryo or young plant are used to distinguish the two large groups of flowering plants. The monocotyledonous and dicotyledonous plant groups produce one or two embryonic cotyledons, respectively. Both internal and external features of both vegetative and reproductive organs can be used to distinguish the monocots from the dicots. The more prominent leaf veins have venation patterns that are referred to as reticulate (netlike) in the dicots and parallel venation in monocots. The number of floral appendages (sepals, petals, stamens, and carpels) can also be used to identify the two angiosperm groups. Dicot species usually produce the separate floral appendages in whorls of fours or fives, while the monocots develop whorls of appendages in threes. Whole number multiples of these base numbers can similarly be used to distinguish the two groups.

Leaves

Leaves, produced as appendages or outgrowths from the vegetative stem apical meristem, have a different tissue arrangement than the stems and roots. The component tissues of the leaf are the epidermis, the mesophyll, and the vascular bundles. The epidermis is formed from the surface cell layer of the stem apical meristem. The epidermis serves a protective function.

Vascular bundles, also called veins, are cylindrical traces of xylem and phloem that diverge from the stem's central stele. The leaf's midvein is the largest medianly positioned vascular bundle. Minor leaf veins develop laterally on either side of the midvein. Both the midvein and minor veins differentiate through the stalk of the leaf (the petiole) into its flat blade.

The blade is composed mostly of mesophyll tissue that develops interior to the enveloping leaf epidermis and surrounds the vascular bundles. Most leaves have two types of mesophyll cells, the palisade and spongy mesophyll. Palisade mesophyll develops near the upper epidermis and is composed of columnar-shaped cells. The spongy mesophyll cells are more spherically shaped and are characterized by the presence of numerous intercellular spaces between adjacent cells. Plants produce most of their food during photosynthesis in cells of the mesophyll tissue.

Although considerable variation in anatomy occurs in the internal leaf tissues of dicots and monocots, the differences are mostly based on environmental rather than taxonomic criteria. There is a distinct difference, however, in the leaf anatomy of certain species, both dicot and monocot, based on whether the first product produced in photosynthesis is a three-carbon or a four-carbon (C₄) compound. C₄ plants, best represented by tropical grasses, develop a thick-walled cell layer around each leaf vascular bundle, called the bundle sheath cells. This concentric organization of surrounding bundle sheath cells is referred to as Kranz (German for "wreath") anatomy. Additionally, the bundle sheath cells contain more organelles such as mitochondria and chloroplasts. Dicot and monocot species in which the first photosynthetic compound produced is a three-carbon compound do not exhibit Kranz anatomy.

Stems and Roots

The arrangement of the vascular tissues within the stele not only distinguishes dicot from monocot species, but also provides a means of identifying the specific vegetative organs. Stems are partitioned into nodes and internodes. Nodes are the locations of leaf development, while the inter-nodes are the distances between the leaves, comprising most of the stem's length. Young dicot internodes usually develop a single ring of vascular bundles when viewed in cross sections. As a result of vascular cambium activity in older woody dicots, the vascular bundles are disrupted and eventually give way to concentric rings of secondary xylem interiorly and secondary phloem exteriorly. The major secondary phloem cell type involved in food conduction is the sieve-tube element. The interiorly produced secondary xylem is composed of tracheids and vessel elements, the water-conducting cells, which have heavily lignified cell walls. The majority of dicot tree species are composed of these thick-walled xylem cells, which represent the wood or lumber used in commerce. A central pith, with peripheral vascular tissues, develops in the center of both dicot and monocot stems, as well as monocot roots. The stems of many monocot species (for example, grain crops) produce a scattered arrangement of vascular bundles that develop throughout the pith and cortex. These vascular bundles do not become disrupted in older stems, since no vascular cambium develops in monocot species. Thus, young and old monocot stems resemble each other in their anatomy. Young and old dicot roots do not develop a central pith but have xylem in their centers. Vascular bundles, so characteristic of stems and leaves, are absent throughout most of the length of roots in both angiosperm groups.

The stems and roots of flowering plants are categorized by their appearance. For example, stem structural types include bulbs, rhizomes, corms, tubers, and stolons that develop underground. Cladodes, tendrils, tillers, and thorns are types of aboveground stems. Root structural types include underground tap and fibrous roots, while buttress and prop roots occur above ground.

Bryophytes and Ferns

Two important groups of nonflowering plants include the bryophytes and the ferns. The bryophytes include the mosses, liverworts, and horn-worts. They lack vascular tissues, and therefore do not develop true leaves, stems, and roots. Since bryophytes produce organs that are similar in structure and function to leaves and stems, these organ names are used, however. Being nonvascular plants, bryophytes ordinarily form a low-growing ground cover, never becoming a conspicuous part of the temperate vegetation. While most cells are thin-walled and spherical, elon-gated cells of conducting tissues develop within the stems of some bryophytes. Leptome and hydrome are analogous to the phloem and xylem tissues of higher plants, respectively. Leaves of bryophytes are produced spirally around the stem. Leaves contain filaments of cells, the photosynthetic lamellae.

Ferns are primitive vascular plants and commonly have complicated arrangements of phloem and xylem tissues within stem and root steles. Most temperate ferns develop elongated underground stems (rhizomes) from which the aboveground leaves (fronds) originate. Developing on the lower side of the rhizome are the slender roots. Fronds, on their inception, are tightly coiled into fiddleheads, termed such because of their resemblance to the carvings on the end of a violin or fiddle. One of the most conspicuous attributes of ferns is the formation of older leaves that are compound, giving them a dissected or lacy appearance. Reproductive structures commonly develop on the underside of fern fronds and are conspicuous due to their brown color in contrast to the leaf's green background. These are the sori in which spores are produced. No development of flowers, fruits, or seeds occurs in the ferns.

SEE ALSO BARK; BRYOPHYTES; CELLS; CELLS, SPECIALIZED TYPES; FERNS; FLOWERS; LEAVES; MERISTEMS; ROOTS; STEMS; TISSUES.

Jan E. Mikesell

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