176 



BURTON EDWARD LIVINGSTON 



If, instead of environmental osmotic pressure, the force tending 

 to withdraw or withhold water from the cell be a force of imbibi- 

 tion, one of capillarity, or one of crystallization, existent in the 

 surroundings (as in the case where the superficial cells of plant 

 roots or of subterranean animals are in contact with a soil of low 

 water content or where water is being frozen at the cell periphery), 

 then the effect of the surroundings is to extract water from the 

 cell walls and thus increase their force of imbibition, which, as 

 we have seen, acts upon the cell in quite the same manner as does 

 external osmotic pressure. Further, with the superficial surface 

 bathed by air, this (according to its relative humidity and rate of 

 circulation) exerts upon the cell the same kind of influence as 

 that exerted by the environmental forces just referred to, and 

 water is extracted from the cell wall, resulting in the same increase 

 in its imbitional force. 



By this sort of summaiy, we see that the environmental force 

 opposed to water intake may be manifest in any one of four forms, 

 all of which are effective in the same way (as far as our present 

 interest is concerned). These four forms are, osmotic pressure, 

 the force of imbibition or capillarity, the force of crystallization 

 of water, and the force of evaporation. In the study of the water 

 relations of any plant or animal, one or more of these external 

 forces must be taken into account, and in the higher plants all 

 three are important. The relative rates of inward and outward 

 movement of water, which determine the water content of cells, 

 tissues and organisms, are directly determined by the magnitude 

 of these forces, on the one hand, and of internal osmotic pressure 

 or protoplasmic imbibition, on the other. 



