It is rather obvious that plants and animals, especially those whose bodies are somewhat complex, are vastly different in their overall morphology. This is an accompaniment of their different modes of existence and simply reflects the principle that structure and function are closely related in organisms.
Thus, the animal tissue that we have studied shows adaptations to an actively motile type of organism whose body cells generally can on a high rate of metabolism.
A somewhat different picture presents itself when a tissue of the complex plant is examined, however. These reflect the sedentary existence carried on by the plant, and adaptations to the structural and functional requirements of this mode of life become obvious with study.
Although certain nonvascular plants exhibit bodies of considerable size and complexity, as a general rule relatively little specialization is seen in their cells. It is particularly significant that they lack tissues which make possible the transport of fluids throughout the plant body. This necessitates a mode of existence for the plant which will enable some of its cells to receive vital materials from other cells by diffusion.
Thus, it is no accident that the nonvascular plants, except for certain algae whose cells are all in relatively close contact with nourishing seawater; do not usually attain much size since they are limited by their lack of specialized conducting tissues. Because the nonvascular plants demonstrate a limited degree of cellular differentiation, therefore, little value is to be gained in studying their development in the hope of discovering any great degree of histological specialization. The vascular plants offer an entirely different picture, however.
Not only does a high degree of cellular differentiation result in the presence of tissues that conduct fluids, but there are adaptations for greater size and more varied habitats. As is the case in animals, there is a considerable degree of variation in the structure of higher plants, but there are tissue types which are common to all.
Embiyogeny of typical vascular plants begins with early divisions of the zygote within the tissues of a parent plant. Although there is little or no histological specialization at this point, a pair of embryonic organs, the root and the shoot, soon become evident.
The root, which exhibits a positive response to gravity, ultimately gives rise to the root system of the plant, whereas the shoot, which is affected oppositely in its growth, serves as the forerunner of the stem and leaves.
In the seed plants, the embryo usually stops growing just after these embryonic organs have developed and becomes surrounded by the tissues derived from the parent plant in the formation of the seed. The seeds of many plants undergo a period of dormancy which seems to be an adaptation for propagation of the species.
There are other species whose seeds remain dormant only until proper conditions of moisture, temperature, and oxygen supply are such as to initiate embiyonic growth. After the root and shoot have grown for a short time, some of their cells differentiate to form the tissues characteristic of the mature plant.
Cells generally remain at about the same level of the plant organ where they begin to specialize. New growth occurs at the tip, which” makes it possible to stud}’ the changes that take place simply by proceeding from the younger to the older cells. If one starts at the tip of a young shoot and proceeds downward by examining both transverse and longitudinal sections, he finds that the first cells encountered are small, undifferentiated cells which are active in division.
These constitute meristematic tissue, a term applied to any tissue in a plant that possesses the ability to undergo active division. As more cells are produced by division, those that are older remain at the same level at which they were produced and begin the process of differentiation. Thus, as one proceeds farther and farther down the stem in his sectioning and study, successive degrees of specialization are seen.