THE GENERAL STRUCTURE 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). 
