Compare and contrast the structure and function of the three simple tissue types: parenchyma, collenchyma, and sclerenchyma.
Parenchyma cells refer to plant cells in the leaf that are usually alive at maturity. They have cell walls that are thin and flexible. They generally have a central vacuole that is large. They have a polyhedral arrangement. They are involved in carrying out photosynthesis, storage functions, and other metabolic functions. Most of these cells have the potential to undergo differentiation generating other cell types in certain conditions. For instance, when the plant is injured and there is a need for repairing and replacing organs. Their large central vacuoles enable them to store water, regulate intake of ions and store waste. Parenchyma cells generally have a variety of functions.
Collenchyma cells are elongated with irregular thick cell walls (Simpson, 2006, p. 413). Like parenchyma cells, they are usually living at maturity. They are involved in providing support to the plant. Their thick cell wall is made up of pectin and cellulose. Collenchyma cells are mainly divided into four types. The first type is the angular collenchyma which is usually thickened at points of intercellular contacts. The second type is the tangential collenchyma whose cells are thickened at cell wall tangential faces and are usually have a regular arrangement. The third type is the lacunar collenchyma which contains cell intercellular spaces. Annular collenchyma is the fourth type, and the cell walls are uniformly thickened.
Sclerenchyma cells are usually the thick secondary cell walls. These cells are often dead at maturity. They are normally found in those parts of the plant that have stopped showing growth. They also provide plants with support. They appear as fibers with bundle caps. There are two types of sclerenchyma cells. The first type is the fibers, which are long, thin and have a secondary cell wall that is regular. The second type is the schlerids, which are short and have an irregular shape.
What is the major function of the epidermis and what role does the three epidermal cell types, ground epidermal cells, trichomes, and stomata, play in serving this function?
Epidermis is the outer layer that covers all the organs of plants and provides protection to plants. It is usually composed of specialized cells that normally differentiate from the basal epidermis cell. One of the specialized cells is the stomata, whose function include plant transpiration and gaseous exchange during respiration and photosynthesis. Stomata usually open and close during these processes. The stomatal opening and closure are due to guard cells. Guard cells are bean-shaped and have chloroplasts. During the daylight, the guard cells open the stomata to allow intake of carbon dioxide by plants. However, at night, the stomata are usually closed by guard cells to avoid water loss by diffusion. The opening and closing process of stomata due to guard cells is facilitated by the presence of chloroplast in guard cells, which carry out photosynthesis to provide energy for the processes.
Another specialized cell of the epidermis is the trichomes. These are narrow and long and grow to the surface perpendicularly. They are of various shapes. They form a hair-like covering on the surface of plant organs like roots, stems, and leaves. The trichomes found in roots are called root hairs and are responsible for water absorption from the surrounding soil. Trichomes on stems and leaves normally enable the retention of water vapor thereby decreasing water loss via evaporation. In some plants, trichomes are responsible for secreting compounds that protect these plants from being fed on by the herbivores ("Plant Defense against Insect Herbivores," n.d.).
Ground epidermal cells of the epidermis contain cutin. They cover plant parts with cuticle which prevent water loss of plant organs.
References
Plant Defense against Insect Herbivores. (n.d.). Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3676838/
Simpson, M. G. (2006). Plant systematics. Amsterdam: Elsevier/Academic Press.
