230 CHEMISTRY OF PLANT LIFE 



thesis of proteins, etc., can proceed, and that the fats, carbohy- 

 drates, proteins, etc., which are synthetized in vegetative cells 

 pass from these to other organs of the plant for use or storage. 

 The obvious explanation for this condition of things in the plant 

 is that protoplasm (and, indeed, this is equally true for prac- 

 tically all known membranes) is not absolutely impermeable to 

 dissolved crystalloids; or, in other words, semi-permeability 

 generally means only that the solvent passes through the mem- 

 brane more readily and more rapidly than do the dissolved mate- 

 rials in it. Even colloidal materials will diffuse through most 

 common membranes, although at so slow a rate that the process 

 is scarcely observable by ordinary methods of study. Hence, the 

 actual permeability of the protoplasm permits the movement of 

 both water and dissolved solids from one part of the organism to 

 amother; but its approximation of semi-permeability produces 

 osmotic pressure and induces freer movement of water than of dis- 

 solved substances, and so provides for turgidity of the cells and for 

 equalization of the water content of different portions of the pro- 

 toplasmic mass. 



It is clear, therefore,. that osmotic pressure plays an important 

 part in the physical mechanism of cell activities and in the reg- 

 ulation of the proportion of water contained in the protoplasm, 

 with its consequent effects upon the chemical reactions which may 

 go on in the cell. 



Actual measurements of the osmotic pressure of plant cell have 

 been made. The results are more or less uncertain, because, as 

 has been pointed out, a plant cell is not a definite quantity of 

 uniform protoplasm surrounded by an ideal semi-permeable mem- 

 brane, but is instead a mass of living matter which is approxi- 

 mately semi-permeable throughout its entire volume and is in a 

 constantly changing condition because of the anabolic and cata- 

 bolic activities which are going on in it; but values have been 

 obtained which show a normal osmotic pressure as high as four- 

 teen atmospheres in the cells of very turgid plants, such as those 

 of some of the green algae. Animal cells probably have an osmotic 

 pressure similar to that of the blood which circulates around them, 

 which is approximate that of seven atmospheres. 



