What are the Types of Tissue in Higher Plant Body?

As a consequence of gradual differentiation, certain tissues come to characterize the mature plant. For purposes of convenience, these may be classified as being either simple or complex, a distinction based on whether a tissue is composed of one cell type or several cell types.

Near the growing tip of a given plant organ, one of the first simple tissues to become differentiated is epidermis, which persists as an external layer, usually one cell thick. It is essentially a protective tissue, and chloroplasts are often absent in all its cells except guard cells.

The most common simple tissue to be four in higher plants is parenchyma, cells of which serve chiefly in synthesis of storage. They are rather thin-walled, usually exhibiting only slight elongation, and function in the living state.

Another simple tissue is collenchymas, which is concentrated chiefly at sites in the plant where strength and support are required while the plant is still relatively young.

These cells generally possess rather thick walls composed of cellulose and they tend to become somewhat elongate. Like parenchyma, they function as living cells. A fourth type of simple tissue is sclerenchyma, cells of which assume two different forms. Some are is diametric and other may become extremely elongate, the latter being called fibres.

Unlike the preceding types of simple tissue, sclerenchymatous cells do not achieve their full potential function until after their protoplasts have disappeared, a process which leaves an extremely thick cell wall surrounding a small lumen formerly occupied by the protoplasmic contents.

The walls of sclerenchymatous cells owe much of their strength to a material called lignin, which becomes associated with the cellulose of young cell walls as they gradually differentiate. This physical characteristic renders sclerenchyma, and especially fibres, very serviceable in lending strength to a given plant part.

The fibres of some plants such as flax and sisal are commercially valuable since they can be used in the manufacture of such commodities as cloth and ropes. In addition to these four types of simple tissues, it should be remembered that merited is always present at the growing tips of plant organs, as well as at various other locations in the plant, and it may be regarded as a fifth type. Complex tissues are of two types, xylem and phloem, which are concerned with the movement of materials in vascular plants.

The functional cells of xylem are either elongate, somewhat tapering units called tracheids, whose ends join in the function of conduction, or vessel elements, which are larger and more uniform, and which become fused together at their ends to form conducting tubes called vessels.

The walls of tracheids and vessels, like those of sclerenchymatous cells, are characterized by the presence of lignin, which lends great strength to them. Lignin may be deposited in the walls of these cells according to a variety of patterns, namely, as rings, spirals, networks, and so on.

These conducting cells of xylem lose their protoplasts upon reaching maturity, and the fluids which they conduct travel through the region of each cell originally occupied by living material. In addition to the tracheids and vessels that have been described, parenchyma cells and sclerenchymatous fibres often are present in xylem, the entire aggregation of cells associated together in the common function of fluid transport being regarded as a single tissue type.

In general, xylem serves to transport water and dissolved materials upward in the plant. The functional units of phloem are called sieve cells, and they are somewhat analogous to the vessel elements of xylem; in many plants, sieve cells are formed as multicellular tubes comparable to xylem vessels. Unlike the latter, however, they retain their protoplasm in the functional state, although nuclei disappear.

Typically, sieve cells are very closely associated with other components of phloem called companion cells. Because there are perforations between companion cells and sieve cells, it has been suggested that the nuclei of the former may serve the cytoplasm of the latter. In addition to these two cell types, phloem is always characterized by the presence of phloem parenchyma and frequently by sclerenchymatous phloem fibres.

In the transport of fluids, which are chiefly dissolved food materials manufactured in the upper parts of the plant and which are carried downward to other parts, the sieve tubes perform a similar function to that of tracheids and vessels in regard to the movement of materials upward in the plant.

Distribution of these tissues throughout roots, stems, and leaves varies greatly among plants, but there are some definite structural patterns that can be identified. Xvlem and phloem tend to develop centrally in roots with absorption occurring only at the level of the root where epidermal root hairs are present.

Water and dissolved inorganic salts pass into the root hairs and from cell to cell inward the xylem, by way of which these materials travel upward in the plant. Sectioning of the roots of flowering plants are characteristically arranged in a circular pattern in dicotyledonous plant stems, whereas in many monocotyledonous stems they tend to occur randomly.

Roots and stems exhibit three general regions, called epidermis, cortex, and steel. The epidermis is typically a single layer of cells, the cortex is that region extending from the epidermis to the beginning of vascular tissues, and the steel is composed of all cells which are thus surrounded by the cortex.

The stele of many roots forms a solid, central cylinder without pith, and the cortex of the root original worm. This implies a certain organization for growth on the part of the animal, apparently built around its longitudinal axis. In general, the power of regeneration is great among those animals whose bodies are relatively simple and becomes more and more limited with the increase of complexity.

Among the vertebrates, it is virtually limited to the healing of wounds, a process that is extremely complex and not always completely successful, particularly when muscular and nervous tissues are destroyed.

This differential in regenerative powers is obviously a reflection of the greater specialization seen in mature cells of the higher animal body; cells such as those of planarian are considerably more unspecialized and versatile than are most cells of the human adult.

Although botanists usually prefer the expression “vegetative growth” in speaking of regenerative process, plants also demonstrate the ability to replace and repair. In fact, even some very complex plants may be propagated because of the tendency of their stems, roots, or leaves to develop into entire plants.

Here again, as in planaria, there is generally an orientation: Perhaps the most important biological principle to be derived from experiments in regeneration is that plants and animals posses a certain organization above the cellular level which results in their being more than a mere sum of their parts, or cells. We have already seen that this is true in the development of animal embryos where the activities of cell groups called organizers have been clearly demonstrated in embryological research.

To return to our former analogy, these organizers are comparable to civic clubs and other organizations within a society whose effect on that society supersedes the influence of any one individual.

Because of the extremely important principles and implications involved, both to biology and to philosophy, the area of research dealing with regeneration and organization is one of the most active and exciting fields of modern biological research.