191 1] William J. Gies 69 



The foregoing Statements emphasize the further fact that cells 

 differ greatly in their immediate need of water, in their resistance 

 to its intracellular removal, and in their power to withstand the 

 changes in its physical condition which are induced by alterations 

 of temperature. Water is an essential " nutrient " at all periods of 

 a cell's life. The largest proportions of water commonly occur, in 

 mammals, in the tissues containing the most active cells. 



Hydrotropism, which is one of the important manifestations 

 of life, is cellular response to the stimulating influences of water — 

 a typical chemotropic reaction. The elastic rigidity of many plants 

 with large proportionate contents of water is due to the tension 

 of their cell walls under the influence of intracellular osmotic pres- 

 sure, colloidal affinity for water, and other forces. 



Water may readily be withdrawn from cells by simple diffusion 

 into such dehydrating agents as alcohol. Water continually 

 evaporates from cells in superficial locations. In plants " wilting " 

 is an indication of such a loss of water and a consequent reduction 

 of turgidity. The presence of water in cells can be immediately 

 demonstrated colorimetrically with such a dehydrating agent as 

 anhydrous copper sulfate. 



Our general knowledge of the facts regarding the condition 

 and significance of intracellular water Warrants the foUowing addi- 

 tional summary : 



Cells contain large proportions of water in the free State. Free 

 intracellular water may be readily removed by evaporation, by 

 ordinary diffusion, and by dehydrating agents. Intracellular water 

 is the general "medium of exchange" inside the cell, and also be- 

 tween the cell and its liquid environment. This free intracellular 

 water contains the soluble intracellular Compounds. It is therefore 

 the mobile medium through which osmotic forces are manifested, 

 which provides for the ingress and egress of diffusible substances 

 (nutrients and waste products particularly), which cooperates in 

 maintaining the functional integrity of the cell colloids, and which 

 provides the carrier for all the vital reaction-exchanges. Intra- 

 cellular water is normally in comparatively stable equilibrium with 

 (A) the water among the adjacent tissue cells, (B) the water in the 



