THE GENEEAL STKUCTUKE OF PLANTS 15 



lowly forms of microbes, is dependent on the access of oxygen 

 for the maintenance of life. 



The oxygen is usually obtained by the plant through 

 the intervention of water. The aquatic plant, whether 

 free-swimming or stationary, unicellular or possessed of a 

 highly differentiated body, absorbs the needed supply from 

 the quantity which is dissolved in the water of the sea, 

 stream, or pool in which it lives. The terrestrial plant conveys 

 it to the protoplasts in solution in the water with which its 

 tissues or its walls are saturated. In such an organism 

 there is, however, need of a special mechanism by means 

 of which the gases of the exterior may obtain access to 

 the living cells in the interior of the mass. 



A third requirement of the plant is food. Here 

 ultimately, again, its dependence is placed upon the water 

 it obtains. The food or the materials from which the food 

 is constructed are absorbed by the plant in solution in 

 water, whether the food material is solid, liquid, or gaseous 

 in the condition in which it is presented to it. 



Another condition is imperative in the case of a plant 

 which is composed of a large number of protoplasts or cells. 

 Not only must each have its own needs supplied, but it 

 must be in a condition to influence others and be influenced 

 by them. In such a plant we have, in fact, a community of 

 individuals, situated differently with regard to the supply 

 of individual and collective needs, and the well-being of 

 the whole community must depend upon the co-operation 

 of all in carrying out the different processes of life. The 

 protoplasts of such a community must therefore be in 

 organic connection with each other, so that such co-opera- 

 tion can be secured. The connection between contiguous 

 protoplasts which are separated by cell-walls is not easy 

 to determine. Special methods of preparation, and the 

 application of particular staining reagents, will show, how- 

 ever, under very high magnification, that the living sub- 

 stance of one cell is continuous with that of its neighbour 

 by fine delicate fibrils which perforate the wall (fig. 17). 



