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 apical meristems, respectively) and within the cambia of woody plants, which grow in girth. The shoot and root apical meristematic 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 of wax 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) or are 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.
Ground Tissues
The ground tissues, the second of the primary developmental tissues, differentiate in the zone of maturation to form tissues called parenchyma, collenchyma, orsclerenchyma.
Continue of the article: Ground Tissues
Procambium
The procambium, the third of the primary developmental tissues, differentiates to form primary xylem and primary phloem as well as the vascular cambium.
Continue of the article: Procambium
|
