THE PILOBOLUS GUN AND ITS PROJECTILE 141 



a pair of scissors, were immersed in it. No plasmolysis took place. 

 The Elodea cells could be plasmolysed slightly with a solution of 

 sucrose having an osmotic pressure of 10 atmospheres but not with 

 one having an osmotic pressure of 8 atmospheres. It therefore 

 seemed reasonable to suppose that the osmotic pressure of the 

 Pilobolus sap is less than 10 atmospheres, a supposition confirmed 

 later by results obtained with the Barger method. No further work 

 with method (2) was attempted. 



(3) Barger's capillary-tube method, which was described in 

 1904, has the merit of being applicable to cell-sap which can be 



z a k 

 » M. n 



IT ■ H 



H 1 I I 



nx: 



Fig. 68. — Part of the apparatus used for determining the 

 osmotic pressure of the cell-sap of Pilobolus longipes 

 by the Barger method. Three capillary tubes attached 

 to a glass slide by Canada balsam. Each tube 

 contains 7 drops : those shown in black, Nos. 1, 3, 

 5, and 7, are drops of sugar solution of known osmotic 

 strength ; those lightly shaded, Nos. 2, 4, and 6, are 

 drops of Pilobolus cell-sap. The tubes are closed at 

 both ends. The whole was kept immersed in water in 

 a Petri dish. Natural size. 



obtained only in small quantities.^ Its essence is the comparison 

 of a solution of unknown osmotic pressure with standard solutions 

 of known osmotic pressures made from a substance of known 

 molecular weight, a series of drops taken alternately from the 

 solution of unknown osmotic pressure and from a solution of known 

 osmotic pressure being introduced into a capillary tube. The 

 vapour pressure of the drops with the lower osmotic pressure is 

 greater than that of the drops with the higher osmotic pressure, in 

 consequence of which water vapour passes from the drops with the 

 lower osmotic pressure and condenses on the drops with the higher 

 osmotic pressure, with the result that the drops with the higher 



^ G. Barger, " A Microscopical Method of Determining Molecular Weights," 

 Journal of the Chemical Society, Transactions, Vol. LXXXV, 1904, pp. 286-324. 



