Anatomy of Plants

The anatomy of plants relates to the internal structure and organization of the mature plant body, especially the vegetative organs 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

Bibliography

Cassab, Gladys I. "Plant Cell Wall Proteins." In Annual Review of Plant Physiology and Plant Molecular Biology, Vol. 49. Russel L. Jones, ed. Palo Alto, CA: Wadsworth, 1998.

Esau, Katherine. Anatomy of Seed Plants, 2nd ed. New York: John Wiley & Sons, 1977.

Fahn, Abraham. Plant Anatomy, 4th ed. New York: Pergamon Press, 1990.

Mauseth, James D. Plant Anatomy. Menlo Park, CA: Benjamin Cummings, 1987.

Moore, Randy, W. Dennis Clark, and Kingley R. Stern. Botany. Boston: Wm. C. Brown Publishers, 1995.

Salisbury, Frank B., and Cleon W. Ross. Plant Physiology, 4th ed. Belmont, CA: Wadsworth, 1992.

Van de Graaff, K. M., S. R. Rush, and J. L. Crawley. A Photographic Atlas for the Botany Laboratory, 2nd ed. Englewood, CO: Morton Publishing Co, 1994.

Anatomy of Plants

Plants are the primary producers in Earth's ecosystem . Plants are autotrophic, meaning that they produce their own food (via photosynthesis), and as a result ultimately produce food for the ecosystem's consumers (such as humans). Understanding plant function is the key to enhancing crop production, preserving plant biodiversity, producing medicines, and much more. However, in order to understand plant function, one must understand plant form.

General Anatomical Organization of Plants

Like animals, plant bodies are made up of a variety of cell types that are organized into tissues. Tissues are organized into organs, and organs function together within systems. Within this hierarchy of structure, emergent properties arise at each level. An emergent property is a characteristic or function that can be found at one level that is not present at lower levels. For example, an individual cell of a leaf cannot perform all of the functions of the leaf, but the cells of the leaf collectively perform the function of a leaf. Therefore, the function of each lower level is best understood in the context of the system in which it exists. For this reason, this article begins by exploring the gross anatomical features of a plant and proceeds to examine the anatomy in progressive detail.

Plants are made up of two organ systems: the shoot system and the root system. For terrestrial plants the shoot system is above ground and consists of a number of organs. These include stems, leaves, and flowers. On the other hand, the root system is most often underground and consists of organs such as roots, underground stems (tubers), and rhizomes.

Each of these organs performs a different function. Stems are support structures and mediate the growth of the plant. Shoot tips contain actively dividing regions called meristems, which produce auxin, a hormone that regulates the growth and shape of the plant. Leaves are the primary sites of photosynthesis, so they are the food production centers of the plant. Flowers are reproductive structures, where eggs and sperm (pollen) are produced and where pollination and fertilization occur. Roots, tubers, and rhizomes are the main system for nutrient and water acquisition and storage. All of these organs are made up of cells that can be categorized into three major tissue types: dermal, ground, and vascular tissue.

Dermal Tissue

Dermal tissue makes up the outer layers of the plant and contains epidermal cells that secrete and are coated with a waxy layer. This waxy coating, the cuticle, prevents excessive water loss from the plant. While the dermal tissue primarily serves a protective role, it also has a variety of other specialized functions depending on the particular organ where it is located.

In leaves, dermal tissue contains specialized cells called guard cells that make up structures called stomata . Stomata facilitate the exchange of gases in the leaf. Carbon dioxide (CO₂) diffuses into the leaf through the stomata for use in photosynthesis, and oxygen (O₂), the waste product of photosynthesis, diffuses out of the leaf through stomata. Stomata are also crucial for water transport through the xylem . Stomatal opening results in the evaporation of water from the air spaces of the leaf. This creates negative water pressure that pulls on the column of water in the xylem. The evaporation of water from the stomata is the main driving force for water transport through the water. In roots, epidermal cells have a specialized structure that facilitates water and nutrient absorption, the main function of the root. Some of the root epidermal cells have long membranous extensions called root hairs that increase the absorptive surface area of the root. Root epidermis also interacts with symbiotic fungi that form mycorrhizae , which increase nutrient absorption.

Ground Tissue

Many different functions are performed by ground tissue including photosynthesis, storage, and support. Ground tissue makes up the majority of the plant structure and is composed of three cell types: parenchyma, collenchyma, and sclerenchyma cells.

Parenchyma cells are the least specialized cells in a plant. These cells are responsible for the production and storage of nutrients. Photosynthesis occurs in the chloroplasts of parenchyma cells in leaves. Parenchyma cells in stems, roots, and fruits have structures that store starch. Most developing plant cells are structurally similar to parenchyma cells. During their differentiation, they become specialized in form and function and lose the potential to divide. Mature parenchyma cells do not usually divide, but they retain the ability to divide and differentiate into different cell and tissue types in the event of an injury to the plant.

Collenchyma and sclerenchyma cells provide structural support for the plant. Collenchyma cells have thick, yet pliable, cell walls. These cells give structural support to newly formed portions of a plant without restricting growth. Collenchyma cells are stacked end on end and are oriented in strands just beneath the epidermis of the young structure. The relatively soft cell wall allows the collenchyma cells to elongate as the structure grows.

On the other hand, sclerenchyma cells provide support to mature plant structures. Like collenchyma cells, they have very thick cell walls. However, the cell walls of sclerenchyma cells contain lignin , a molecule that makes the cell wall hard. This provides strength to the cell wall, but restricts the ability of the cells to elongate and grow. Since a sclerenchyma cell functions solely to provide structural support, many sclerenchyma cells are actually dead at functional maturity. The cell membrane, protoplasm (cytoplasm) and organelles are gone, leaving only the rigid cell wall that serves as a scaffolding system for that structure.

Vascular Tissue

Vascular tissues make up the organs that transport water, minerals , and food throughout the plant. Vascular tissue can be divided into two functional units. Xylem transports water and minerals from root to shoot. phloem transports nutrients (such as sugar and amino acids ) from leaves and other production sites to roots, flowers, stems, and other tissues that need them. The cells that make up vascular tissue are unique in their structure. Their specialized characteristics allow them to transport material through the plant efficiently while providing structural support to the plant.

Xylem tissue contains two types of cells: tracheids and vessel elements. Like sclerenchyma, both of these cell types are dead at functional maturity and therefore lack protoplasm. Tracheids are long, thin cells that have tapered ends. They overlap on another, and water passes from tracheid to tracheid via small pores. Vessel elements are shorter and are stacked end to end, forming more of a tube structure. Water flows in the tube by passing through perforated end walls between cells.

Phloem tissue is made up of two different types of cells: sieve tube members and companion cells. Sieve tube members are the main conducting cells, and are named for the sievelike areas along their cell walls through which the phloem sap moves from cell to cell. Unlike cells of the xylem, sieve tube members are alive at functional maturity, but do not have nuclei. For this reason, companion cells are closely associated with sieve tube members. These cells do have nuclei and serve to support the sieve tube members. The cytoplasm of sieve tube members and companion cells is connected through numerous pores called plasmodesmata. These pores allow the companion cells to regulate the content and activity of the sieve tube member's cytoplasm. Moreover, the companion cells help to load the sieve tube members with sugar and the other metabolic products that they transport throughout the plant.

see also Algae; Angiosperms; Bryophytes; Cell Wall; Conifers; Fruits; Gymnosperms; Leaves; Meristems; Mycorrhizae; Pteridophytes; Roots; Shoots; Translocation; Water Movement in Plants

Susan T. Rouse

Bibliography

Atlas of Plant Anatomy. <http://atlasveg.ib.usp.br/English/>.

Moore, Randy, et al. Botany, 2nd ed. New York: McGraw-Hill, 1999.

Plant Anatomy Images. University of Rhode Island. <http://www.uri.edu/artsci/bio/plant_anatomy/images.html>.