Crafts et al. 



96 



Water in Plants 



are prepared, and into them are drawn, first, a droplet of unknown (sap), 

 followed by an air space, then by a droplet of sucrose solution of known 

 concentration, and so on. Each tube then contains alternating layers of 

 sap, air, sugar solution, air, sap, etc. (Figure 32). The tubes are sealed in 



Fig. 32. — The Barger Halket method for determining 

 the osmotic pressure of plant sap. Capillary tubes contain 

 alternate columns of standard sucrose solutions and sap. 

 In this figure the black columns represent stock solution, 

 the clear columns sap. Each tube contains stock solution 

 of a different concentration. 



a flame and mounted on a slide. Several tubes are prepared for each de- 

 termination, each varying with respect to the concentration of the sugar 

 solution. By means of a. binocular microscope and micrometer, the length 

 of the sap columns is measured before and after equilibrium has been at- 



FiG. 33. — Osmotic pressure determination by the method 

 of Ursprung and Blum (1930). Standard reference solutions 

 are contained in capillary tubes c, attached to a cover glass d 

 by plastic cement p. The cover glass with attached capillaries 

 is sealed on top of a shallow dish by grease f. The depression 

 in the dish contains the expressed sap I. 



tained. The results are placed on a graph, and that concentration that pro- 

 duces no change in the length of the sap columns is considered to be iso- 

 tonic with the sap, i.e., having equal vapor pressure and osmotic pressure. 

 The method depends upon the diffusion of water vapor through air, which 

 acts like a semi-permeable membrane. Though the method is tedious and 

 subject to error from mixing of the droplets and movement of films along 



