Influence of the Medium 133 



In the first series of cultures several modifications of 

 Knop's solution were used. This solution consists of: Ca 

 (NO 3) 2, four parts; MgSO^, KNO3, and K^HPO^, each 

 one part, with the addition of a trace of iron. In order 

 to determine whether a change in the concentration of this 

 solution would affect the plant in a chemical or physical 

 way, four modified solutions were made up, each being defi- 

 cient in one of the four constituent salts. The deficient salt 

 was reduced to one-tenth its normal quantity, and, the 

 decrease in osmotic pressure thus brought about having been 

 calculated, a sufficient amount of each of the three other 

 salts was added to increase the pressure by an amount equal 

 to one-third of the calculated decrease. Thus were obtained 

 four solutions, all of which had the same osmotic pressure, 

 but each of which was deficient in one salt. 



The calculations for the pressure corrections were made 

 both by the now obsolete method of De Vries, and by assum- 

 ing that, in the concentrations used, ionization was complete. 

 Solutions made by both methods gave the same results upon 

 the plant, and after a first trial the second method of cal- 

 culation was exclusively used. During the summer of 1901 

 the pressure of nearly all these solutions was tested by the 

 freezing-point method. A table of the results so obtained 

 will be found in the second paper cited on this subject. 

 The error introduced by the assumption of complete ioni- 

 zation was found to be too small to interfere with the 

 accuracy of the results in any degree, the discrepancy 

 between the real and the calculated pressures lying well 

 within the limits of the threshold of stimulation for this 

 alga. 



The cultures showed that all four modified solutions, and 

 the normal Knop's solution also, influence the plant in exactly 

 the same manner. The form of the alga is always deter- 

 mined by the osmotic concentration of the medium, and is not 

 affected by the varying proportions of the constituent salts. 



