186 



Pub. Puget Sound Biol. Sta. 



Vol. 2, No. 45 



there was sufficient time for the plant to become permanently adjusted to 

 the last concentration before measuring the effect of this last dilution on 

 its osmotic pressure. The freezing point of this water was then taken in 

 order to compare it with that of the plants growing in it. This enables 

 us to determine the osmotic surplus maintained by the cells throughout 

 the adaptation process. 



From the depression of the freezing point (A) the osmotic pressure 

 is easily computed from the formula P^22. 4X^/1. 85. The explanation 

 of this formula is simply that A for a gram-molecular solution of a non- 

 electrolyte is 1.85° C, and this depression at 0° C. corresponds to an 

 osmotic pressure of 22.4 atmospheres. For each 1.85° lowering of the 

 freezing point, therefore, there is an osmotic pressure of 22.4 atmospheres. 

 If the pressures are computed from the relation between molar concen- 

 tration of cane sugar solutions and their osmotic pressures as established 

 by Morse and Frazer (1905), different results are obtained. But, as has 

 already been pointed out (Garry 1915), direct measurements of cane sugar 

 solutions give pressures in excess of what they should be according to 

 theory. Harris and Gortner (1914) have published a table of freezing 

 point depressions and corresponding osmotic pressures in agreement with 

 results computed from the formula P=22. 4 X A/1.85. The osmotic pres- 

 sures given below are read directly from that table. 



Owing to the fact that after removing a half dozen kelps or so from 

 the tubs every day, the supply began to run short, the freezing point de- 

 terminations were discontinued when the water was a little more than 

 half fresh in order that the remaining kelps might be left and the experi- 

 ment continued to see how much fresh water they would stand. The 

 writer (1916) found that young kelps tolerated a proportion of 55% 

 fresh water without extreme injury. In this later attempt one plant sur- 

 vived the 28 changes and remained alive several days in entirely fresh 

 water. It was firm in texture and very evidently alive, but the color had 



Table 1. SJiowing the osmotic adaptation of Nereocystis to decreasing 

 concentrations of surrounding sea water 



