ABSORPTION AND MOVEMENT OF WATER 111 



The pressure of the cell-sap and of the cell brought 

 about by this means is called turgor, and it is evidently a 

 very important factor in maintaining the form of the cell 

 and, therefore, comprehensively, of the organs and of the 

 individual. The turgor or turgescence of the cell tends to 

 be maintained by the continued absorption of water; but 

 unless the absorption continually exceed the loss of water, 

 by evaporation or otherwise, there will be no turgor, the 

 plant will be flabby, wilted. Absorption of water can be 

 continued only by keeping the density of the cell-sap always 

 greater than that of the water outside. 



The production, control, and maintenance of this physical 

 condition is accomplished in part by the living protoplasm, 

 in part, in multicellular plants, by the osmotic absorption 

 of water from one cell by another. In the latter case, water 

 is drawn off by physical means only, and in obedience to 

 unavoidable physical law, by the cells that need it. In the 

 former case, the living protoplasm, by what it takes from 

 and gives to the cell-sap in respiration, nutrition, and excre- 

 tion, maintains the greater density of the cell-sap, that is, 

 maintains the higher proportion of dissolved matter to 

 water than prevails outside the cell. 



Upon the composition of the cell-sap depends the absorp- 

 tion of the substances dissolved in the water outside the 

 cell. We have already seen that a dissolved salt will pass 

 through a permeable membrane into another volume of 

 water which contains less or none of it, and that the rate of 

 osmosis (or, in this case, of absorption) will depend upon 

 the salt, the difference in the proportions of the salt in the 

 two volumes of liquid, upon the nature of the membrane, 

 etc. The cell and by means of it, the plant will absorb by 

 osmosis those salts which occur in the soil and in water in 

 proportions larger than in the plant. For example, com- 

 mon salt will be absorbed by a root-hair or by an algal cell 

 until in the cell-sap there is the same proportion of salt to 

 water as in the solution outside the cell. When there have 

 come to be the same number of molecules of salt in equal 

 volumes of cell-sap and outside water, there will be no fur- 

 ther absorption of salt, and although the movements of 



