V 



298 BOTANICAL GAZETTE [October 



elucidation of these difficult relationships would require further 

 study by the methods of water and sand_ culture, which may be 

 subjected to more rigorous control. It is thought that some of 

 the data pertaining to the cell sap of plants from these various 

 soil, water, and sand culture experiments are worthy of a brief 

 discussion at this time. 



The soils were kept in large tanks under controlled conditions 

 as described by Stewart (6). The technique of the water and 

 sand cultures will be described elsewhere. The procedure was 

 designed to place, so far as possible, no limitation on the growth 

 of the plant other than the variables under investigation. The 

 nutrient solutions were made to have a composition similar to 

 that of the soil extracts with respect to the important elements. 

 Various concentrations of both acid and neutral reaction were 

 employed. In each concentration acid and neutral solutions had 

 an almost identical osmotic pressure, and the relation between 

 the various ions was very similar. The reactions were governed 

 by the hydrolysis of the various potassium phosphates used. 



The procedure employed in obtaining the cell sap consisted in 

 cutting the plant into small pieces, freezing first in brine, then in 

 a carbon dioxide ether bath, and finally pressing out the sap as 

 thoroughly as possible through cheesecloth. It is realized that 

 the exact concentration of the sap is dependent upon the tech- 

 nique employed, but the results are comparative and the general 

 magnitudes, which are of interest in this discussion, are probably 

 not far different from those obtainable with other methods of 

 extraction. Osmotic pressures were determined by the freezing 

 point method, conductivity measurements in the usual manner 

 at a temperature of 25° C. The hydrogen ion concentrations 

 were measured with the aid of the hydrogen electrode, using the 

 apparatus described by Sharp and the author (5). The data for 

 the freezing point depressions, conductivity, and hydrogen ion 

 concentration of the various cultures are summarized in table I. 



Effect on osmotic pressures 



Considering the total osmotic pressures first, it will be noted 

 that both the tops and roots, ei£her in the acid or neutral solutions, 

 reflect the concentration of the nutrient solution, although no very 



