Perspective

The eukaryotic cell is composed, with a few exceptions, of both a living protoplast, the site of cellular metabolism, and an enclosing cellulosic wall of one or more layers (Fig. 3.1). While not alive as a structural unit, the wall is commonly traversed by living components, plasmodesmata, which connect adjacent protoplasts and thus facilitate communication between, and the integration of, cells within a tissue. All plant cells possess a protoplast during development, and in many it persists throughout the life of the plant. Some cells, however, do not achieve their ultimate functional state until the protoplast dies as, for example, a specialized water-conducting cell such as a vessel member.

The protoplasts of all plant cells are basically similar, but may differ in relation to the function of the mature cells. For example, the protoplast of a parenchyma cell in the outer cortex or in a leaf will contain many chloroplasts since a major function of these cells is photosynthesis. In contrast, a cell of the pith (a storage region) in the center of the stem may lack chloroplasts but will contain unpigmented plastids in which starch is synthesized (amyloplasts). The protoplast of an immature vessel member, however, destined to die, may contain no plastids at all, or plastids of a highly modified type.

Each cell protoplast is characterized by the potential for the development of an entire organism (see Steward et al., 1964). This total potentiality is, however, rarely achieved under normal conditions.