CYTOLOGY The structure and development of the vegetable cell so far as the protoplasm and the nucleus are concerned are considered in their broader aspects in the article CYTOLOGY.
The recognition of the fundamental similarity of the cells of plants and animals and the identity in structure and functions of animal and vegetable protoplasm which formed the starting point in the study of cytology are conceptions which subsequent investi gations have only broadened and confirmed. Nevertheless this agreement, whether it be in the minute structure of the protoplasm or in the details of nuclear mitosis, is accompanied by peculiarities in the cell structure of plants which have had a profound influence in the evolution of the vegetable body. Of these the most import ant, because far-reaching in their effects, are the presence of plastids in the protoplasm and the existence of the cell wall. In the higher plants the egg is necessarily a naked cell, but when fertilization is accomplished the new cell secretes around itself a membrane—a cell wall—before it begins to grow and divide, and the myriad cells to which it gives rise in the course of its further development are from the first enclosed in a similar membrane.
The continuous development which is such a characteristic feature of the structure of the higher plants depends on the presence of persistent embryonic regions or growing points at such places e.g., as the tips of stems and roots. This enables us to study in these regions the formation of new cells, and to trace their development and modification to form the adult tissues of the member to which they belong.
The individual cells appear rectangular in form and consist of a mass of protoplasm containing a large nucleus and separated from the neighbouring cells by a delicate membrane. It is unneces sary to refer to the detailed structure of the protoplasm and the nucleus, since these have already been fully described (see CYTOI, OGY), but with suitable treatment it is possible to distinguish in the protoplasm of these meristem cells the rudiments of definite protoplasmic organs, the plastids. The nature, purpose and func tion of these important bodies will be considered below: for the present it is sufficient to say that they are differentiated portions of the protoplasm which perform definite functions.
From such embryonic cells all the tissues of the plant body are formed by processes of growth and differentiation. Growth in volves an increase in size of the individual cells which may in some cases result in an enlargement to many thousand times their original volume. This enlargement does not however necessitate a proportional increase in all the parts of the cell. Increase in size is mainly the result of active extension of the cell wall. The protoplasm does not increase in volume in the same proportion and hence cavities appear in it which are filled with a watery fluid —the cell sap. As the wall enlarges these spaces or vacuoles increase in size and number and eventually coalesce to form one large central vacuole in the protoplasm which thus becomes re stricted to a comparatively thin layer closely applied to the cell wall (fig. I, E). The nucleus, which undergoes little change during this process or may even be reduced in size, is at all times em bedded in the protoplasm and thus comes to lie close to the cell wall or in some cases it may be slung by delicate protoplasmic strands in a more or less central position in the cell. It is often possible to observe a circulation of the material in these strands, definite streams moving from the cell wall towards the nucleus and vice versa. It is, however, uncertain as to whether this is a general phenomenon.