1919 Hurd; on Osmosis in Nereocystis 191 



water by the plant in its effort to bring about a state of equilibrium be- 

 tween its cell sap and the outside water. This therefore consists partly 

 in equalizing the osmotic pressure inside and out by a dilution of the cell- 

 sap. Table 4 shows the weights obtained. 



We can safely conclude from this experiment that increased intake 

 occurs in plants in water of subnormal salinity and so helps to enable 

 them to adapt the pressure of their cells to the lowered pressure outside. 

 In this they act as we would expect any living membrane to do. We 

 might also expect that the amount of imbibed water as shown by increased 

 weight would vary inversely with the concentration of the sea water. 

 The preceding data apparently show this to be true ; however, the evi- 

 dence is weakened by the inconsistency in the results obtained in the 60% 

 and 100% fresh water. The low percent of increase in these two cases 

 may be due to the death of the bulbs and hence the failure of the osmotic 

 exchange to occur, or to some other and unknown reason. On account of 

 lack of time the experiment could not be repeated, so unfortunately the 

 inconsistent figures must stand for the present unexplained. 



6. CONCLUSIONS 



1. The osmotic pressure of Nereocystis luetkeana is intimately re- 

 lated to the concentration of the sea water, decreasing as the water be- 

 comes less saline. 



2. Nereocystis will not tolerate a sudden change to fresh water, but 

 can be made to endure water 100% fresh if the adaptation is gradual 

 enough to allow adjustment of the cell sap concentration to that outside the 

 plant. 



3. The osmotic pressure of Nereocystis in normal sea water as de- 

 termined by the freezing point method is 22.72 atmospheres, and was 

 lowered without killing the plants to 12.52 atmospheres in water 17/28 

 fresh, at which point the experiment was ended. 



4. The osmotic pressure of Puget Sound water averaged 19.2 at- 

 mospheres. 



5. Throughout the adaptation of the cells of Nereocystis to fresh 

 water they maintain an average osmotic surplus, of 3.62 atmospheres. 



6. The lowering of the osmotic pressure as the plant adapts itself 

 to increasing dilutions of sea water is brought about ( 1 ) by the outward 

 diffusion of salts and (2) by the increased intake of water. 



I wish to acknowledge with sincere gratitude the assistance of Dr. T. 

 C. Frye, who placed the facilities of the Puget Sound Biological Station 

 at my disposal; and the helpful suggestions of Dr. V. E. Shelford of the 

 University of Illinois. 



