94 DIFFUSION AND OSMOTIC PRESSUEE 



phere is not so essential, nor is it essential that an extensive 

 surface be exposed at all. A comparatively small area will 

 suffice for the evaporation of the gaseous products of respi- 

 ration, etc., or these may be allowed to diffuse outward into 

 the solutions of the substratum. Thus, in colorless parasites 

 and saprophytes are found reduced surfaces covered with 

 leathery tissues which contain few or no openings. How- 

 ever, water is so plentiful in some habitats that many of the 

 forms found in such places have never acquired any special 

 protection against evaporation. 



In aquatics the effects of evaporation are not present; 

 absorption and transmission of water take place by direct 

 diffusion, perhaps mainly through the roots, however. 



Since both cell wall and protoplasm are permeable to 

 water, this substance will diffuse into a cell when, for any 

 reason, its diffusion tension is less within than it is outside. 

 Thus, if the surrounding medium is a very dilute solution, 

 all cells must absorb water until an equilibrium is estab- 

 lished between the expanding solutes within the vacuole and 

 the elastic cellulose wall without. By imbibition the cell 

 wall is kept saturated with water also, so that there is direct 

 water communication between adjacent cells even where 

 there is no protoplasmic connection. 



This form of water absorption is universal in all organ- 

 isms. It makes no difference how complex the form, the 

 individual cells stand in the same relation to water external 

 to them as does the Myxomycete plasmodium moving about 

 in its moist substratum. Of course in the higher forms the 

 cells are fixed in position. In the latter the important 

 special condition is that here the moist substratum in which 

 any cell lives is often the adjacent living tissue of the same 

 plant. In a complex tissue, if one cell loses water faster 

 than its neighbors, water diffuses from them into it and 

 equilibrium is maintained. 



