482 



BOTANY 



FIG. 455. Median section of the apex of a shoot of the 

 Redwood (Sequoia sempervirens) ; d, dermatogen ; pb, 

 periblem ; pi, plerome. 



original cell. In the 

 lower Thallophytes, 

 generally, cell-divi- 

 sion is immediately 

 followed by growth 

 of the cells to the 

 normal size. In more 

 complex plants, how- 

 ever, this is not the 

 case, and growth and 

 cell-division are not 

 necessarily immedi- 

 ately connected. In 

 these plants there is 

 a special region, usu- 

 ally the tip of the 

 shoot, where cell- 



division is especially active, and very often all of the oells may be 



traced back to a single initial cell at the 



apex (e.g. Fucus, Polysiphonia, Chara). 



Sometimes, as in most Spermatophytes, 



the active tissue (meristem) at the apex 



cannot be referred to the activity of a 



single initial cell (Fig. 455). The small 



meristematic cells have relatively large 



nuclei, delicate walls, and usually 



abundant cytoplasm, indeed, they are 



structurally much like the fully devel- 

 oped cells of some low Thallophytes. 

 The active multiplication of the 



meristematic cells at the apex of a 



growing organ is accompanied by very 



little increase in bulk. The region of 



most active growth (using this word 



in the sense of increase in size) is not 



the apex of the growing organ, but a 



zone some distance behind it (Fig. 456). 



In this subapical zone the volume of the 



cells increases many times, and is due 



largely to the great increase in the 



amount of water taken up by the cell, 



which is very greatly stretched. The FIG. 456. Two seedlings of 



amount of cytoplasm in these cells is Faba, illustrating the unequal 



very little more than in the young cell, zones . f w * h ba , ck of th t e 

 / . . J apex of the elongating root. 



and it forms a thin layer surrounding (After SACHS.) 



