76 TEE BOTANICAL MAGAZINE. [Vol. 1 xxxiv. No. 402. 



system of coordinates on a plane surface. By this means, equally- 

 spaced points in the diagram represent various different solutions of a 

 single type. All contain the same salts and have the same total 

 concentration, but each one differs from all the others in its salt 

 proportions.^ 



Little serious attention has thus far been given to the partial 

 concentration of the very important solutes, oxygen and carbon- 

 dioxide, in such nutrient solutions as are here considered. Experiment- 

 al solutions are generally prepared very carefully with regard to their 

 salt contents, but their oxygen and carbon-dioxide contents are mainly 

 left to chance. In the experiment here reported no attention was 

 given to the partial concentrations of these two non-salt solutes, ex- 

 cepting to plan the cultures so that all jars were of the same size, 

 shape, etc., all solutions were employed in the same volume and with 

 the same extent of aerial surface, and all were renewed at the same 

 time intervals. 



Similarly, the temperature of the nutrient medium (and of the 

 roots surrounded by it) is of great importance in determining the 

 manner and rate of growth of the culture plants. In experiments of 

 the kind with which we have to deal, the temperature of the nutrient 

 solution follows rather closely the air temperature of the place where 

 the cultures stand, especially when the direct heating effect of sunshine 

 upon the culture jars is largely prevented by the use of an opaque 

 jacket around each jar. It must be remembered, however, that the 

 growth of the culture plants is externally influenced not by the root 

 environment alone but also by the surroundings of the leaves, etc., 

 which are not in the solution but are bathed by the air. Con- 

 sequently, the kind of growth that will be obtained by the use of 

 any given plant with any given solution is not to be predicted from 

 a knowledge of the chemical make-up and temperature of the solution ; 

 to these solution conditions must be added all the influential conditions 

 that are active in the aerial surroundings of the plant. Among these 

 aerial conditions may be mentioned, especially : air temperature, air 

 humidity, air movement, the carbon-dioxide content of the air, and 

 the group of conditions generally treated as those of light (radiation 



1) The whole subject here reviewed is clearly presented in a " Plan for co- 

 operative research on the salt requirements of representative agricultural plants," 

 published by the Special Committee on Salt Eequirements of Representative Agri- 

 cultural Plants, of the Division of Biology and Agriculture of the U. S. National 

 Research Council (Baltimore, 1919), copies of which may be obtained from the 

 Committee. 



