410 MANUAL OF BOTANY 



spot to the periphery. A nucleus is a constituent of all cells 

 when they are young, though in some few oases it disappears 

 later. It is always embedded in the protoplasm, of which it is 

 a specially differentiated portion. 



Not only does the living substance construct the substance 

 of the plant originally, but it is the part by which the organism 

 is able to place itself in harmony with its environment. It 

 assimilates the food the plant requires and carries out the 

 chemical processes incident to its life ; it receives impressions 

 from without and regulates the response the plant makes to 

 these impressions, both by internal and external movements 

 or changes of position ; finally it carries out the reproductive 

 processes. 



Looking at the arrangement of protoplasm in the cell, or at 

 its environment in the free condition, we notice especially its 

 very close relation to water. The free-swimming zoospore is 

 naturally saturated with the latter, being in the fullest contact 

 with it. The young cell enclosed in its ceU-membrane speedily 

 shows a tendency to accumulate water in its interior, and gradu- 

 ally drops appear, which lead ultimately to the formation of a 

 vacuole always fall of liquid. The healthy protoplasm is thus 

 always in contact with water. Indeed the molecular constitution 

 of protoplasm, as far as we know it, lends itself to this relation, 

 for the apparently structureless substance is always saturated 

 with it. It is only while in such a condition that the 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 mani- 

 fest their dependence upon water for the maintenance of life. 

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

 obstacle to the absorption of water, and bo even those cells which 

 are Hving in streams or ponds usually possess a vacuole. Cells 

 without a membrane, such as the zoospores already many times 

 mentioned, can more readily absorb water from without, and 

 hence they are not vacuolated to the same extent as the former 

 ones ; indeed, many of them have no vacuole. This cavity being 

 always filled with liquid, the protoplasm of the cell has ready 

 access to water, as much so indeed as the cell which possesses 

 no wall. The vacuole contains a store which is always available. 



We have seen that the presence of water is necessary to the 

 life of the cell, and that a store of it is usually contained in its 



