Influence of the Medium 127 



extreme care must be used to have them of exactly the same 

 osmotic concentration as those of CuSOi, which were pre- 

 viously used. Then, if the organism lives in all the dilute 

 solutions, no matter of what substance, it may be concluded 

 that the determining factor is one of osmotic pressure. If, 

 on the other hand, the organism lives in the concentrated 

 solutions of cane sugar, glucose, KNO3, NaCl, etc., but dies 

 even in a weak solution of HgOlg or CuSO^, it must be 

 concluded that the conditions of the medium which deter- 

 mine life or death are of a chemical nature. It is thus pos- 

 sible to analyze the effects of a solution by using a number 

 of different solutes. 



The primary effect upon an organism of an increase in 

 the osmotic concentration of the surrounding medium is 

 extraction of water, that is, it is a drying effect. The pri- 

 mary effect of a decrease in the osmotic concentration is the 

 reverse, it adds water to the organism. 



II. presentation of material 



Following is a review of the several lines of experimental 

 evidence which have been brought forward in connection 

 with the question of the effect of variations in the concen- 

 tration of the medium upon the living being. Since there 

 is so little to be presented, the work upon both animals 

 and plants will be included. The material at hand will be 

 discussed under four heads: (1) The effect upon growth, 

 (2) the e'ffect upon reproduction, (3) the effect upon move- 

 ment, and (4) the analogy between the effects of high 

 osmotic pressure of the medium and those produced by 

 other water-extracting processes. 



a) Variations in the osmotic pressure of the surround- 

 ing medium: their influence upon the growth and form of 

 organisms. — A number of observations upon various organ- 

 isms have been made, all tending to the general conclusion 



