The body plan of a plant is very different from that of most animals. Terrestrial plant bodies are anchored in a growing medium, which has an enormous influence over the form and behavior of plant tissues.
Growth and Protective Tissue
Meristematic tissues in plant bodies are responsible for the growth that results from an increase in cell number. In the meristems, individual cells divide to produce pairs of daughter cells which have the ability to divide further or to enlarge and differentiate.
The meristems are located at the ends of branches and roots (shoot apical meristems and root apicalmeristems, respectively) and within the cambia of woody plants, which grow in girth. The shoot and root apicalmeristematic tissues produce cells that account for the lengthening of the shoots and roots.
The primary developmental tissues are in a region called the zone of elongation. These developmental tissues are distinguished from meristematic tissues by the larger size of their cells and by their locations.
Three primary developmental tissues are produced by the shoot and root apical meristems. They are the protoderm, the ground tissues, and the procambium. As these primary developmental tissues mature, they will ultimately differentiate into the metabolically more active portions of the plant.
In a region called the zone of maturation, the cells begin to take on the characteristics of mature, functioning tissues. The protoderm differentiates to form the epidermis, a mature tissue protecting the surfaces of plant parts which do not have secondary vascular tissues. The epidermis is made of cells which have one side in contact with the environment (air, water, or soil). The other side is in contact with other cells in the plant body.
Epidermal tissue in contact with air is usually protected by a layer ofwax called the cuticle. It may also be covered with hairs,water-filled cells, poison-filled barbs, or even digestive glands. These specialized structures provide protection from particular environmental conditions and may even serve as paths for the absorption of nutrients in the case of carnivorous plants.
The underlying tissues must have access to atmospheric gases for their metabolic activities. To accomplish this, the epidermal tissues are punctuated by pores which open and close (stomata) orare permanently open (lenticels). Epidermal tissues in contact with the ground require a different kind of protection. These tissues may secrete mucus, which protects growing underground structures.
There are epidermal cells that fall off the plant body to provide a lubricating barrier between the rest of the plant and the soil. Finally, the epidermal tissues nearest the root tip may be covered with long subcellular hairs that contribute significantly to the root’s ability to absorb water and minerals.
Epidermal tissues of plant organs that normally grow in water are less likely to bear the specialized structures of epidermal tissues from aerial or subterranean parts. These cells are often more like parenchyma cells of ground tissues than they are like epidermal cells of subterranean or aerial structures.
The ground tissues, the second of the primary developmental tissues, differentiate in the zone of maturation to form tissues called parenchyma, collenchyma,or sclerenchyma. The parenchymous tissues are the primary site of cellular metabolism. The organelles of parenchyma cells in different parts of the plant vary so that they can accommodate differences in metabolic functions.
Cells of leaf parenchyma and some stemparenchyma have large numbers of chloroplasts to carry out photosynthesis. Stem and root parenchyma cells have amyloplasts, organelles that store starch. Chromoplasts in the parenchyma of flower petals contribute to the color of the flower petals.
|The ground tissues|
Parenchyma cells producing large quantities of protein have more ribosomes than those specialized for starch storage. Reproductive parenchyma cells may have unusual nuclear characteristics that prevent these tissues from competing with the developing embryos for nutrients or space.
Parenchyma fill the inner parts of leaves, stems, and roots. These cells have large, water-filled vacuoles. The water pressure from these vacuoles provides much of the rigidity of the body of non woody plants. When a leaf is limp, its parenchyma cells are usually depleted of water.
Many of the chemicals that give plants their unique tastes or pharmaceutical characteristics are produced and stored in parenchyma. For example, the bulk of a carrot root (especially outside the central core), the mass of a potato tuber (which is actually a unique form of stem), and much of a lettuce leaf are all made of parenchyma.
Collenchyma cells are similar to parenchyma cells in many ways. They use water pressure to provide support. However, they are normally found near the surface of stems and leaves. Collenchyma cells have a unique pattern of cell-wall thickening that allows expansion in diameter but not in length.
This makes collenchyma especially suited to providing support for soft-bodied plant parts that have completed much of their longitudinal growth. Collenchyma cells rarely provide bulk to plant structures. Instead, they form thin sheets just below the epidermis and outside much of the parenchyma.
Because collenchyma is thin, it has a smaller volume than parenchyma and contributes less to the metabolism of the plant organs. It may nevertheless support some of the photosynthesis of the plant, and it provides textures to the organs as well.
Sclerenchyma cells occur throughout the body of the plant and include three types of cells: elongated fibers; branched sclereids, resembling a three-dimensional jigsaw puzzle piece; and globular stone cells. All three cell types have heavy, secondary cell walls and have lost many organelles. Sclerenchyma is a type of differentiated tissue that functions when its cells are dead.
Fibers support plant organs in the same way as does collenchyma, but because the secondary cell walls of sclerenchyma cells resist longitudinal and latitudinal expansion, they are not common in growing tissues. Their rigidity helps to supply support even when tissues are water-stressed, but it also limits the potential for the organs to expand in girth or length.
Fibers, sclereids, and stone cells all provide protection against predation. The gritty texture of a ripe pear, the shell of a nut, and the strings of a coconut husk are all composed of sclerenchyma cells and promote the wear and breakage of predators’ teeth and other chewing structures.
The procambium, the third of the primary developmental tissues, differentiates to form primary xylem and primary phloem as well as the vascular cambium. The vascular cambium produces cells that differentiate into secondary xylem and secondary phloem. It also regenerates the supply of cells in the vascular cambium.
An example of xylem is the woody tissue at the center of most trees. (Palm trees are a notable exception.) Smaller bundles of xylem are found in the roots, stems, and leaves of most plants, even when they are not woody.
Xylem tissues are made of four cell types: fibers and parenchyma cells (which also occur in sclerenchyma and parenchyma) and xylem vessel elements and tracheids (which are found only in the xylem). These cells work in concert to move water upward through the plant. The xylem vessel elements and tracheids provide the actual channels for the movement, and the fibers serve largely as physical supporting structures.
The parenchyma cells are responsible for some lateral movement in the xylem tissues. These parenchyma cells also have the ability to revert to a meristematic condition, providing a mechanism for the xylem to replace damaged cells. Tracheids and vessels have unusual, patterned secondary cell walls that resist the physical stresses involved in moving xylem sap.
The cell organelles are lost before the vessels and tracheids are functional. The sap moves through a channel where the body of the cell had been before itwas lost. Xylemis another tissue that contains cell types that function when they are dead.
An example of phloemis the tissue on the inside of the bark of most trees (again, palm trees are an exception). Smaller bundles of phloemare found in the roots, stems, and leaves of most plants, even when they are not woody.
Phloem tissues are made of fibers, parenchyma cells, sieve tube elements, and companion cells. These four cell types work in concert to move sugars, other organic molecules, and some ions throughout the body of the plant.
The sieve tube elements (or sieve cells, in some plants) provide the actual channels for the movement. The fibers serve largely as physical supporting structures. The parenchyma cells are responsible for some lateral movement and also provide a mechanism to replace damaged cells.
Sieve tube elements and sieve cells have unusual, perforated cell walls whose appearance indeed resembles a sieve. Many of the cell organelles are lost before the sieve tube elements and sieve cells are functional. The phloem sap moves through living cells, but they resemble no other cells in the plant.
The companion cells (which in some plants are called albuminous cells, to indicate a different developmental origin) are similar to parenchyma cells, but they provide substantial metabolic support to the sieve tube elements and sieve cells.
These cells function together: The companion cells could live independently, while the sieve tube element could not, but the important function is carried out by the sieve tube element.
Most species that grow in girth are woody, and the wood of woody plants is composed almost entirely of secondary xylem. The bark of woody plants is made of phloem and corky layers. There are two principal cambia, the vascular cambium and the cork cambium.
Both contribute to the increase in girth. The vascular cambial tissues produce the cells that will differentiate to form the secondary xylem and phloem of woody species. The cork cambium produces the corky cells on the outside of the bark.