THE STRUCTURE OF PLANTS 



31 



protein, and finally ends by making the most complex of all, 

 protoplasm. But, except in rare cases, all plants must have 

 compounds to start with ; they cannot make any of these 

 nutrients or protoplasm from chemical elements. 



And thus we learn that food materials are gradually 

 changed by protoplasm into living substance like itself. To 

 this process is 

 given the name 

 assimilation 

 (Latin, ad = to 

 = similis =like) . 

 As a result 

 of the process 

 of assimilation 

 the amount of 

 protoplasm of 

 course increases 

 and the cell 

 grows. Were 



this process to continue indefinitely, cells would become 

 large in size. This, however, does not occur; for when 

 a cell reaches its normal size, the nucleus divides, and the 

 halves separate from each other to form two nuclei. The 

 cell-body now divides into two parts, and cell-walls are 

 formed between the two cells (Fig. 7). Thus are produced 

 two cells, each having its own nucleus, and these in turn as- 

 similate, grow, and divide. In this way the number of cells 

 increases with the growth of the plant. 



A B C 



FIG. 7. Cell division. 



A, cell before division ; B, cell with divided nucleus ; C, single 

 cell that has divided into two cells. 



