The evolution of the fully-erect land habit in plants has been the result of a capacity for great variation in cell types to meet the structural and functional requirements consistent with success in the terrestrial environment.
Three main types of cell differentiation have been evolved from the basic pattern to meet these requirements. First, mechanical and supporting tissue has evolved to counteract the lack of body support offered by the land conditions. Secondly, highly specialized conducting or vascular tissues have arisen to enable the spatially separated absorbing tissue (root system) and the photosynthetic tissues (leaves) to be in direct and continuous communication with one another. The vascular system found in all higher plants allows for conduction of materials to and from all parts of the plant and in many instances may lend additional support to the mechanical tissues; it may even supplant them entirely. Thirdly, outer specialized epidermal layers protect against desiccation by the drying atmosphere.
It is necessary to point out that the vague beginnings of the differentiation of cells according to such needs have been encountered in the gametophytes of mosses, but that no gametophyte of any plant shows the kind of tissue differentiation shown by the sporophyte of the vascular plants. It is the sporophyte generation which has evolved as the fully-adapted land plant, but from what origin is entirely unknown.
When studying the shape and size characteristics of the various kinds of plant cells as seen in thin sections, it is important to realize that the microscope can indicate structure in one plane only. It is therefore necessary to examine transverse and longitudinal sections of the same tissue. Macerates are very useful in the study of isolated cells.
Types Of Tissues
It is usual to describe mature plant tissues(vascular plants) under the following main functional categories:
- Parenchyma or ground tissues.
- Mechanical and supporting tissues.
- Vascular and conducting tissues.
- Outer protective tissues.
It is useful to consider them in this way because it emphasizes the differences in structure which may be associated with special functional requirements.