20 



THE RELATION OF DESERT PLANTS TO 



The suggestion is due to Whitney and Cameron (1903, p. 54) that 

 we may attack this subject by means of an artificial root hair, in the 

 form of the ordinary osmometer, consisting of a semipermeable mem- 

 brane precipitated in porous clay and filled with a solution of known 



osmotic pressure. Following this suggestion 

 a number of porous clay cylinders or cups 

 were obtained for the preparation of osmotic 

 cells. These were designed especially for this 

 work. They are hollow cylinders of unglazed 

 porcelain, 12.5 cm. in length and having an 

 internal diameter of 2 cm. and a thickness of 

 wall of about 3 mm. One end is closed and 

 rounded, the same thickness of wall being 

 retained here as at the sides. The other end 

 is open and grooved within so as to give good 

 surface of contact for a rubber stopper, while 

 the thickness of the wall is doubled here for a 

 distance of 2 cm. back from the edge, the 

 thickened portion terminating in an external 

 shoulder. A working drawing for one of these 

 cells is shown in figure 3. 



In preparing the osmometers, the precip- 

 itation membrane of copper ferrocyanide de- 

 vised by Pfeffer (1877) was employed. The 

 cylinders were boiled in distilled water to expel 

 air and allowed to cool under water. They 

 were then filled with n/10 potassium ferrocy- 

 anide solution and were placed upright in a 

 beaker containing copper sulphate solution of 

 equivalent strength, the surface of the exter- 

 nal solution being just below the upper edge 

 of the porcelain. In this condition the cells 

 were allowed to stand from two to five days, 

 at the end of which time a good semipermeable 

 membrane of copper ferrocyanide was usually 

 found to have been formed within the porous 

 wall. When the membrane was judged to be 

 complete the cells were removed, thoroughly 

 washed with water, and filled with a 1.5 molecular solution of cane sugar. 

 The opening was tightly closed by a rubber stopper with a single per- 

 foration, through which passed a glass tube of about 4 mm. bore. The 

 tube extended above the stopper a distance of about 50 cm. In the act 



FIG. 3. Mechanical drawing for 

 porous clay cylinder for use 

 in osmotic experiments and 

 in evapori meter. 



