SOIL MOISTURE AND TO EVAPORATION. 71 



tracing the contour with a pencil. They were then placed in solutions 

 of potassium nitrate of different concentrations and left for half an 

 hour. At the end of this period they were removed and again placed 

 on the tracings which represented their original contour, note being 

 taken as to whether the effect of the salt solution had been to increase 

 or decrease their curvature or to leave it practically the same as at the 

 beginning. Since the epidermal layer is practically nonabsorptive for 

 water and also gives it up with great difficulty, while the cut surfaces of 

 the storage tissue absorb and give out water very readily, an increase in 

 curvature denotes an absorption by the latter tissue and a decrease 

 denotes an extraction of water by the external solution. Thus those 

 solutions which caused no change in curvature are to be regarded as 

 isotonic with the cell sap of the cortex, those in which curvature 

 increased are of lower concentration than this sap, and those in which 

 curvature decreased are of higher concentration. Of course this method 

 is based upon the general assumption that potassium nitrate fails to 

 penetrate the protoplasmic membranes of these cells. 



A large number of tests of the form just described were carried out 

 with several different individual plants, and the results showed that 

 the cell sap of the storage tissues just beneath the epidermal layers has 

 a concentration which is equivalent to that of a potassium nitrate solu- 

 tion having a strength of from n/9 to n/5. That is, this sap has an 

 osmotic pressure of from 3.9 to 7 atmospheres. The middle point 

 between these extremes of pressures is very close to the value obtained 

 by the method of the freezing-point, 5.38 atmospheres, so that the two 

 methods are in fair agreement. 



Similar tests were made with strips from the flattened internodes of 

 Opuntia Engelmannii, and gave n/6 as the approximate concentration 

 of potassium nitrate which is isotonic with the sap of their storage 

 tissues. This is equivalent to about 5.9 atmospheres and is seen to be 

 approximately the same as the pressure found in the case of Cereus, 

 but somewhat greater than that found in Echinocactus. The osmotic 

 pressures exhibited by these plants are not markedly higher than the 

 author has often observed in the cortex of scapes of Taraxacum and 

 stems of Ricinus seedlings. It is not nearly as high as that observed 

 by Sutherst (1901) with the freezing-point method in the case of a num- 

 ber of common agricultural plants. This author found, for instance, 

 that the sap of the green stalks and leaves of celery have a pressure of 

 1,284.25 cm. of mercury. (See in this regard Livingston (1903), p. 85). 



All of the cactus juices experimented with contained considerable 

 amounts of mucilaginous material. While such substances do not alter 

 the freezing-point of the solution and probably have no effect upon the 



