PlantPlant, member of a large and diverse group of organisms sharing certain common features, but difficult to separate absolutely from all other living things (see BIOLOGY). Two characteristics stand out because of the sharp contrast with ANIMALS. Plants are primarily autotrophs (self-feeders), using light energy from the SUN to synthesize organic molecules from inorganic precursors (photosynthesis). They are stationary organisms and obtain their energy while fixed in one place. These properties are reflected in the structure of a typical land plant (vascular plant) which is organized into 2 basic systems, shoot and root.
The shoot system (stem and leaves) grows upward into the light and is the site of photosynthesis; the root system penetrates the soil, anchors the plant and absorbs necessary water and minerals. Both systems are potentially unlimited in growth, thus providing for the immobile plant a means of adjustment to the environment.
Cell StructureA plant's immobility is readily understood when its cells are examined. Unlike animal cells, each plant cell is enclosed in a boxlike wall, the main structural component of which is cellulose. Furthermore, the walls of adjacent cells are held together by a cementing substance (but intercellular spaces occur frequently, especially where several cells meet at their edges). All plant cells have a relatively thin, outer primary wall, capable of extension during cell growth.
Certain supporting and conducting cells have an inner, often relatively thick secondary wall which is incapable of growth. The secondary wall does not cover the primary wall completely but is interrupted by small pits or by more extensive gaps. Clearly, nerve and muscle tissues, the bases of animal motility, could not be constructed of such cells. The meristems continually add new, functionally active cells at the growing tips, apparently accomplishing the result achieved in animals by cell turnover. Thus, there is a steady replacement of leaves as the shoot grows and of absorbing root hairs near the root tip.
Inside the cell wall, the protoplasmic contents are bounded by a differentially permeable membrane like that surrounding the animal cell. Plants lack an organized nervous system but have plasmodesmata connecting almost all living cells. These are fine strands of protoplasm (bounded by membrane) that extend through the primary wall and provide cell-to-cell continuity. When a secondary wall is present, plasmodesmata are restricted to the pits.
The cytoplasm (generalized cell protoplasm) contains a nucleus and several organelles of diverse structure and function. Conspicuous among these are plastids, which in photosynthetic tissues contain chlorophyll and are known as chloroplasts. A further, distinctive characteristic of plant cells is the presence of fluid-filled vacuoles bounded by a membrane similar to the cell membrane. Small and often numerous in immature cells, they enlarge and fuse during cell growth so that a single, central vacuole, sometimes traversed by cytoplasmic strands, occupies most of a mature cell.
Types of Cells
Plant cells have many different forms, but those encountered in vascular plants fall into a few classes.
Tissues and OrgansThe diverse cells of the plant body are organized into tissues, some relatively homogeneous, others more complex. On the basis of structure and function, tissues may be grouped into 3 systems: vascular (conducting) system; dermal (protective) system; and fundamental (metabolic) system. These systems occur, in somewhat different configurations, in each major plant organ (root, stem and leaf).
In the root, the vascular system consists usually of a central core of xylem, with radiating ridges, and phloem, located in troughs between the ridges. Around the phloem is a layer of parenchyma (pericycle) in which branch roots originate. Surrounding the pericycle is the endodermis, cells of which have a band of suberized material in the transverse and radial walls that restricts the passage of materials into or out of the vascular system. Tissues inside the endodermis are sometimes referred to as the stele.
The fundamental system is represented by the parenchymatous cortex and sometimes by a core (pith) in the centre of the xylem. The cortex is bounded by the epidermis. In a zone just behind the root's growing region, certain epidermal cells extend outward in projections (root hairs) which are extremely important in absorption from the soil. The root cuticle is usually very thin, particularly in the absorbing region.
In the stem of SEED PLANTS, the vascular system consists of interconnected bundles of xylem and phloem, the phloem outside the xylem. These bundles are continuous with the vascular supply of the leaves, and one or more diverge into each leaf at the level of its node (point of attachment to stem).
In dicotyledons (plants with 2 embryo leaves), the bundles form a ring around a central pith; in monocotyledons (plants with one embryo leaf), they are scattered throughout the centre of the stem, embedded in fundamental tissue. The cortex is often photosynthetic. The stem is bounded by an epidermis with stomata.
In the leaf, the petiole (stalk that supports the blade) contains one or several vascular leaf traces embedded in fundamental tissue, which often includes collenchyma. In the blade (lamina), the vascular system is subdivided into a network of veins and veinlets serving all parts of the photosynthetic tissue. The fundamental system (mesophyll) is composed of chloroplast-containing parenchyma with extensive intercellular spaces.
Often 1 or 2 layers of columnar cells, the palisade layer, occur just below the upper epidermis and above the more open, spongy mesophyll. Stomata are usually more numerous in the lower epidermis. The veins traversing the mesophyll are surrounded by a compact layer of parenchyma, the bundle sheath, often associated with substantial amounts of collenchyma (sclerenchyma in larger veins).
At the tip of each shoot and root are functioning regions of continued growth (meristems). Meristems originate in early embryonic development. The fertilized egg (zygote) develops into many cells which, like those of the animal embryo, begin to specialize for a function in the adult body. Two groups of cells remain unspecialized or embryonic (eg, capable of continued cell division). These become the first shoot and root apical meristems.
The shoot apical meristem initiates stem tissues, produces outgrowths that develop into leaves and initiates primordia of lateral branches just above the leaf axil (junction of leaf and stem). With suitable stimulation, the shoot apical meristem may be transformed to give rise to a flower, inflorescence or cone, thus relinquishing its capacity for unlimited growth.
The root apical meristem initiates root tissues and a protective covering over itself (root cap). It forms no appendages comparable to leaves or branches; branch roots arise internally, emerging some distance behind the root tip.
Although an entire plant body can be formed by the shoot and root apical meristems, a substantial supplement is often provided by additional or secondary meristems, especially in TREES and shrubs. The vascular cambium and cork cambium contribute additional tissues to the vascular and dermal systems respectively, a further example of cell replacement by addition. The vascular cambium is a layer of meristem situated between the xylem and phloem. By longitudinal division of its cells parallel to the surface of the stem or root, it forms secondary xylem or wood to the inside and secondary phloem to the outside. In trees and shrubs, this activity may continue for years. Wood is one of Canada's major natural resources.
In herbaceous plants cambial activity is greatly restricted, or absent (in most monocotyledons). Even without a cambium, plants such as palms and tree ferns can build up a massive body and maintain a long life span. The continued expansion of the vascular system internally cannot be long contained by the epidermis, the rupture of which would have serious consequences if the dermal system were incapable of replacement.
However, the cork cambium, located near the surface, produces a periderm (bark) composed largely of suberized cork cells which restrict water loss. Lenticels, which perforate the periderm with loose, spongy parenchyma, accomplish aeration. These openings are not controlled but may be sealed by development of cork and reopened by further production of spongy tissues.
Parenchyma cell are roughly equidimensional in shape, have only thin primary walls and carry on most of the plant's metabolic activities (eg, photosynthesis, storage).
Collenchyma cells also retain active protoplasmic contents, but are elongated with thickened primary walls, often unevenly distributed around the cell's circumference. They combine support with flexibility.
Sclerenchyma cells have thick secondary walls and provide rigid support. When mature they are usually dead, containing no protoplasmic contents. Elongated sclerenchyma cells are called fibres; and more nearly equidimensional ones are sclereids.
Tracheid and Vessel Cells
Tracheid and Vessel cells are the conducting cells of the xylem (water-transporting tissue). Both are dead at maturity and have secondary walls (either a continuous wall with pits or in the form of rings, spirals or a network). Tracheids are elongate and spindle-shaped. Water passes between them through pits or other gaps in the secondary wall. Vessel cells vary from elongate to barrel shaped and are superimposed one above the other to form vessels. The end walls of a vessel cell are perforated, leaving no barrier to water flow within the limits of a vessel.
Sieve elements are conducting cells of the phloem (tissue that transports organic solutes). Unlike xylem elements, these are living cells, but the protoplasm has undergone substantial alteration (usually including loss of nucleus). In flowering plants, sieve elements are superimposed to form sieve tubes and are connected by plates through which enlarged intercellular connections extend.
Epidermal cells form a surface barrier against water loss. They resemble parenchyma but the outer wall is often thickened and impregnated with cutin, a substance largely impervious to water. A layer of cutin, the cuticle, is also deposited on the outer surface. Certain epidermal cells are modified in shape and function as guard cells for the stomata (minute openings on leaf or stem surfaces). Others may form unicellular or multicellular outgrowths (hairs or trichomes).
Cork cells are dead when mature, have walls modified by deposition of suberin and reduce water loss.