THE GENERAL STRUCTURE OF PLANTS 13 



It is only by its powers of responding to such impressions 

 that the whole organism is able to place 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 sub- 

 stance 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 water. 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. 



The constancy of the occurrence of the vacuole in the 

 cells of the vegetable organism is itself an evidence that 

 such cells are completely dependent upon water for the 

 maintenance of life. The cell-wall, though usually 



FIQ. 16. ADULT VEGETABLE 

 CELLS. x 500. (After 

 Sachs.) 



h, cell- wall; p, protoplasm; 

 k k', nucleus, with nuoleoli j 

 s' s, vacuoles. 



