28 PLANT LIFE 
renders possible the production of those 
almost infinitely complex substances which 
form the very substratum of life itself. 
CHAPTER III 
EVOLUTION OF CELLULAR STRUCTURE IN 
SIMPLE PLANTS 
EmpnHasis has already been laid on the 
circumstance that the plant cell, owing to 
the presence of its investing skin of cellulose, 
is only able to absorb and use substances 
capable of diffusing through the membrane. 
Consequently food from without can only 
reach the protoplasm in solution. Salts 
and other solid food materials are invariably 
absorbed in a state of solution, and the same 
is true of gases, such as oxygen and carbon 
dioxide, as well. 
But water has other functions to discharge 
within the plant, besides that of serving as a 
vehicle for the intake of nutritive materials. 
It serves to maintain the protoplasm itself 
in that characteristic semi-fluid condition 
