21] 



CHAPTER I. THE CELL. 



87 



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 protoplasmic contents, con- 

 taining, 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 pro- 

 cesses of repro- 

 duction, what 

 have been termed 



FIG. 36. Cells and their structure. A Young cells from tbe 

 ovary of Sj/mphoricarpu* rticemosus (x 300); B cells from an 

 older ovary of the same plant ( x 300) ; C and D from the fruit 

 of the same plant (x 100); 7i cell-wall; p protoplasm; fc 

 nucleus; fefc nncleolus; vacnole. In C there is a f>injrle 

 large vacuole, the whole of the protoplasm forming the parietal 

 layer. In D there are several vacuoles, and the nucleus lies in 

 a central mass of protoplasm connected with the parietal layer 

 by numerous strands. 



such as zoospores, 

 gametes, sperma- 

 tozoids, and 

 oospheres (see p. 

 59), each of which 

 is simply an unit 

 of protoplasm 

 without any cell- 

 wall, though the 

 zoospores event- 

 ually 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 O001 of a 



