CORKELATIVES OF WATER CONTENT AND EXCHANGES 263 



exchange and velocity quotients can be measured upon many more 

 organisms or parts. In chosen conditions and materials, permea- 

 bility coefficients, coefficients of osmotic flow, or velocity quotients, 

 or all of them, may prove to be constant with time, or with concen- 

 tration gradients, or with water increments. Economy of numeri- 

 cal evaluation is achieved by discovering the conditions within 

 which constancy prevails ; in other circumstances, quantitative com- 

 parisons are still allowed by use of the same coefficients. 



Where the factors of pressure gradient and of surface area of 

 exchange are known, the ' ' water-time ' ' system of four variables is 

 expanded into a system of six variables. If this "water- time- 

 gradient" system were to be studied exhaustively, [(n-l)^ + 

 (n-l)]/2 or 15 combinations of coordinates (taken 2 at a time) 

 would be obtained instead of the mere 6 combinations of the re- 

 stricted system. 



The uniformities among diverse organisms in which net water 

 is passing across an external boundary to and from a liquid medium 

 (osmosis) are roughly as follows: (1) Upon transfer to a medium 

 of higher concentration than the original medium, water loss is 

 faster. (2) Simultaneous exchanges of solutes modify the rates of 

 exchange of water. (3) Rates of water exchange are influenced by 

 temperature, specific ions and molecules, orientation of concentra- 

 tion gradient, "non-solvent" components of organisms, durations 

 of exposure, and other recognizable factors. (4) Zero water ex- 

 change may occur without equality of osmolar concentrations 

 inside and outside, as is characteristic of many organisms in fresh 

 water which nevertheless are permeable to water. (5) Modifica- 

 tions of rates of water exchange with water load are not described 

 by a single permeability coefficient in any species. It may be con- 

 cluded that equilibration of water content involves more factors 

 than those customarily recognized in the one equation of permea- 

 bility. 



'§ 96. Body size, age 



In all the data presented in this investigation, water contents 

 and exchanges were measured relative to body weight. Up to the 

 present point an implicit trust has been placed in this basis, when 

 comparing diverse individuals of one species and of different spe- 

 cies. I now inquire what influences this basis has on the relations 

 found both within and among species, and what consequences lead 



