94 PART II. THE INTIMATE STRUCTURE OF PLANTS. [ 22. 



superficial growth of the cell- wall. Hence, since the protoplasm 

 mast remain in contact with the cell-wall at all points, the result- 

 is that cavities, the vacuoles, are formed which become filled with 

 cell-sap (Fig. 46 B). The vacuoles, small at first, increase with 

 the growth of the cell, and may fuse together to a greater or less 

 extent owing to the gradual withdrawal of more and more of the 

 protoplasm into the now extensive parietal layer. 



Cells such as these are examples of the kind of cells which com- 

 pose the succulent parts of plants, such as the cortex of stems and 

 roots, the tissue of leaves, succulent fruits, etc., in fact the bulk of 

 the actually living tissues of the plant. In the higher plants it is 

 generally the case that a considerable number of the cells of the 

 body eventually lose the whole of their proper contents, contain- 

 ing, in fact, nothing but air or water ; such are cork-cells and 

 vascular wood-cells. Such structures are no longer living cells, 

 but are merely their skeletons, and are of use only in virtue of the 

 mechanical properties of their cell-walls. 



On the other hand, there are frequently found in connection 

 with the processes of reproduction, what have been termed 

 primordial cells, such as zoospores, zoogonidia, gametes, sperma- 

 tozoids, and oospheres (see p. 69 and p. 80), each of which is 

 simply an energid, or rarely a complex of several energids (e.g., 

 zoogonidium of Vaucheria), without any cell-wall, though the 

 zoospores and zoogonidia eventually secrete a cell- wall when they 

 come to rest, as do also the oospheres after fertilisation. 



The size and form of the cell vary widely. While some cells 

 are so small that little more than their outline can be discerned 

 with the help of the strongest magnifying power (about O'OOl of 

 a millimetre in diameter), others obtain a considerable size (from 

 O'l to O5 millim.), so as to be distinguishable even by the naked 

 eye (e.g., in pith of Dahlia, Impatiens, Sambucus). Many grow 

 to a length of several centimetres, as the hairs upon the seed of 

 Gossypium (cotton) ; and if ccenocytes be included, such as the 

 laticiferous tubes of the Euphorbiaceee, the Siphonaceous Algae, 

 and the Phycomycetous Fungi, very much larger dimensions in 

 length are attained. 



The Form of such cells as constitute an entire individual, or 

 exist independently, not forming part of a tissue (e.g. spores), is 

 generally spherical, or ovoid, or cylindrical. The various organs 

 of highly organised plants consist of many varieties of cells, and 

 even in the same organ cells lie side by side which are of very 



