STRUCTURE OF PLANTS. 45 



also nitrogen. It can be made more evident by applying 

 a drop of nitric acid to the cell, when, being of a very 

 similar nature to albumen, it coagulates and contracts from 

 the action of the acid, so as to lie as a loose sac within the 

 cell. (PL i, Fig. 3.) 



The origin of the cell is by no means yet quite clearly 

 made out ; only this much is certain, that a peculiar little 

 body appertaining to the primordial utricle and called the 

 cell-nucleus, (PL I, Fig. 1, a.) plays a very important part 

 in it. 



These cells in the course of their development become 

 crowded closely together, and thus form the whole mass 

 of the plant, the cellular tissue, which however may be 

 divided into three principal classes of tissue, according to 

 the different forms of the cells, and more especially 

 according to their importance to the life of the plant. 



But before we betake ourselves to the consideration of 

 these three tissues, we must make a somewhat close 

 acquaintance with the metamorphoses which the cell may 

 pass through during its life. We may regard the cell 

 as a little independant organism, living for itself alone. 

 It imbibes fluid nutriment from the surrounding parts, 

 out of which, by chemical processes which are constantly 

 in action in the interior of the cell, it forms new substances 

 which are partly applied to the nutrition and growth of 

 its walls, partly laid up in store for future requirements, 

 partly again expelled as useless and to make room for 

 the entrance of new matters. In this constant play of 

 absorption and excretion, of chemical formation, trans- 

 formation and decomposition of substances, especially 

 consists the life of the cell and since the plant is nothing 

 but a sum of many cells united into a definite shape also 

 the life of the whole plant. 



