Tuesday, March 31, 2009

Know the Plant Tissues

Vascular plants have distinctive cell types, all of which are surrounded by a cell wall of cellulose fibers and other molecules secreted by the cells. Just as in animals, cells are organized into tissues that perform different functions, but plants do not have organ systems like those of animals. The tissues of plants are grouped into three basic kinds: ground, vascular, and dermal. Meristem is a special embryonic tissue.

Plants differ from animals in that the tips of roots and stems, called apical meristem, remain embryonic and retain the ability to form new structures (e.g., leaves, stems, flowers, and roots). Hormones secreted by meristem cells are transported elsewhere in the plant; meristem is in part analogous to the endocrine system in animals.

Ground Tissues (Simple tissues):
Ground tissues or the simple tissues include parenchyma, collenchyma, and sclerenchyma. Thin-walled parenchyma cells have a variety of functions such as photosynthesis, starch storage, and secretion; they retain the capacity to divide and are important in repair of damage. They form the large part of the bulk of various organs such as stem, root, etc. In some parts they are modified to perform some special functions. For example:

This is a single-layered tissue that covers the whole plant body. It protects the internal part from infection and loss of water. This layer of cells has a waxy coating on the surface, which is secreted by the cells. This waxy layer is called cuticle, which helps to reduce the water loss.

These types of parenchyma cells are found in the leaves between the two epidermal layers. These are specialized for carrying out photosynthesis. Parenchyma cells containing chlorophyll are also known as chlorenchyma. If the cells of the mesophyll tissue are tightly packed without air space, they are known as palisade parenchyma or mesophyll; if a lot of air space is present it is called spongy parenchyma. Endodermis, pericycle, and companion cells, etc. are also an example of modified parenchyma cells.

These cells resemble parenchyma cells but are characterized by the presence of extra cellulose at the corners of the cells. Their walls are thickened and made strong with cellulose and pectin. Collenchyma cells help strengthen the plant parts in which they occur. Celery strings are an example.

Sclerenchyma cells have very thick secondary walls that are commonly impregnated with lignin, which makes them quite rigid. The lignified sclerenchyma of flax plants is made into linen threads for weaving, sewing, and paper making. Wood is made of lignified xylem cells. The hardness of a coconut shell or a peach pit is caused by lignified cells. Ground tissues are analogous to the supporting connective tissue and skeletal elements in animals. Sclerenchyma cells act as supporting elements in plants. Mature sclerenchyma cells can't elongate. The two types of sclerenchyma cells are fibers and sclereids. Fibers are long, slender, and tapered cells that occur in bundles. Sclereids are shorter than fibers and shaped irregularly. Nutshells and seed coats are composed of sclereids. Sclereids scattered among the soft parenchyma tissue of the pear give it a gritty texture.

Complex Tissues:

Complex tissues consist of more than one type of cells. Vascular tissues of plants include xylem and phloem; this is the plant's circulatory system. Xylem and phloem are the complex tissues.
Xylem consists of four types of cells-trachieds, vessel elements, parenchyma, and fibers. Trachieds are single cells that are elongated and lignified. At maturity, trachieds cells are dead and form interconnected tubes throughout the plant. Vessels are long, tubular structures formed by the fusion of several cells end to end in a row. They conduct water and dissolve nutrients that the plant absorbs from the soil; their thick, sclerified walls allow them to give mechanical support to the plant. Wood is made of xylem cells. Xylem parenchyma has thin cellulose cell walls and living contents similar to the typical parenchyma cells. Xylem fibers are shorter and thinner than trachieds and have much thicker walls.

Phloem consists of tubular cells modified for translocation. These tubular cells have interconnected cytoplasm and they conduct other solutes, chiefly nutrients (e.g., carbohydrates) from areas of food production such as leaves to areas of food storage such as tubers. There are five types of cells in phloem. They are sieve tube elements, companion cells, parenchyma, fibers, and schlerids. Sieve tubes are long tube-like structures formed by the end to end fusion of sieve tube elements. Adjacent to the sieve tube elements lie the companion cells with dense cytoplasm. Phloem parenchyma is similar to the ordinary parenchyma cells and the phloem fibers are like the sclerenchyma fibers.

Dermal tissues include epidermis and cuticle. The epidermis is a continuous layer of tightly packed cells. It is usually coated with a cuticle of waxes embedded in a fatty substance; this is analogous to keratinized outer layer of skin, including your own, in animals that live on land. Leaf epidermis is perforated by stomata for gas exchange between the photosynthetic mesophyll (parenchyma) and the surrounding atmosphere. Thus, leaves function in part like lungs. All these tissue types-both simple and complex tissues-are distributed all over the plant parts, but their position and orientations are different in different organs like stem, roots, leaves flowers, fruits, etc.

Tags: Bio Technology, Bio Genetics , Plant Tissues

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