THE GENERAL STRUCTURE OF PLANTS 13 



itself in harmony with its environment. Finally, it carries 

 out the processes of reproduction. 



The primary needs of a plant are fairly simple. If we 

 study the life and the behaviour of one of the free-swim- 

 ming organisms of which we have already spoken, we see 

 that its first requirement is water. In this it lives ; from 

 this it draws its supplies of nutriment and into this it pours 

 forth its excreta. The arrangement of the protoplasm in the 

 cell in one of the higher plants points to a similar need. If 

 we regard the arrangement whether in the young or the 

 adult cell, we notice particularly the very close relation of 

 the protoplasm to water. -The young cell enclosed in its cell- 

 membrane speedily shows a tendency to accumulate water 

 in its interior, and gradually drops appear in its substance 

 which lead ultimately to the formation of a vacuole always 

 full of liquid (figs. 15, 16). This store of water in the 

 interior of a cell is of almost universal occurrence in the 

 lowly as well as the highly organised 

 plant. The constitution of proto- 

 plasm, so far as we know it, depends 

 upon this relation, for the appa- 

 rently structureless substance is 

 always saturated with it. It is only 

 while in such a condition that a cell 

 can live ; with very rare exceptions, 

 if a cell is once completely dried, 

 even at a low temperature, its life is 

 gone, and restoration of water fails 

 to enable it to recover. 



FIG. 16. ADULT VEGETABLE 



The constancy of the occurrence CELLS, x 500. (After 



Of the VaCUOle in the Cells Of the 



vegetable organism is itself an evi- 



dence that such cells are completely 



dependent upon water for the maintenance of life. The 



cell-wall, though usually permeable, yet presents a certain 



obstacle to the absorption of water, and so even those 



cells which are living in streams or ponds usually possess a 



cell-wall : p, protoplasm ; 



nu ' 



