202 TEXTBOOK OF BOTANY 



term is needed to designate diffusion through a particular kind of mem- 

 brane under special conditions with special results. 



There are certain conditions in plant cells under which the number 

 of molecules of water moving from outside the cell through the film of 

 protoplasm into the vacuole is greater than the number moving outward 

 from the vacuole. As a result, the water in the vacuole increases in vol- 

 ume and thereby presses against the protoplasm and cell wall. These 

 structures become stretched and extended, and the whole cell becomes 

 larger and firmer, or turgid. When the enlarging cells occur in masses as 

 in plant organs and press against each other, the mutual pressure among 

 all the cells causes the whole plant to become rigid or turgid. When 

 these processes and conditions become reversed, the plant wilts. If there 

 is any advantage in adding the word osmosis to our consideration of 

 diflFusion in plants, it lies in limiting its use to the diffusion of water 

 through certain kinds of membranes, such as protoplasm, when certain 

 other conditions exist. What are these conditions, and what is peculiar 

 about these membranes? 



The concentration of water in the vacuole is a factor in osmosis. When 

 the diffusion of water is inward through protoplasm into the vacuole, 

 one may infer from the general laws of diffusion that the water inside 

 the vacuole is less concentrated than the water outside the cell. This 

 condition actually occurs when the concentration of dissolved substances 

 (sugars, salts, organic acids) in the water enclosed in the vacuole ex- 

 ceeds the concentration of dissolved substances in the water surrounding 

 the cell. 



At room temperature (20° C.) there is a definite number of molecules 

 of water in each cubic centimeter of pure water. If some sugar is added, 

 the water then appears to occupy more than a cubic centimeter of space. 

 The reader will undoubtedly explain this increase in volume by visualiz- 

 ing the molecules of sugar as diffusing or moving among the molecules 

 of water and jostling them farther apart. Molecules of sugar now occupy 

 spaces formerly occupied by some of the molecules of water, and there 

 are fewer molecules of water in a cubic centimeter of this solution than 

 there are in a cubic centimeter of pure water. The presence of the sugar 

 molecules decreases the concentration of the water. Obviously the more 

 sugar present the more dilute the water will be. In solutions of this sort 

 the water is said to be the solvent, and the sugar the solute. 



Differentially permeable membranes are factors in osmosis. The nature 

 of the membrane, however, is as important in osmosis in plant cells as 



