296 BOTANICAL GAZETTE [NOVEMBER 
increase or decrease in both of them together might influence 
the plant differently from a corresponding change in only one. 
Under this head there are no less than ten possibilities, six 
taking the salts by twos, and four by threes. (2) Further, the 
stimulus may be of a physical nature and due, not to the change 
of salts at all, but to a change of osmotic pressure upon the 
living cell. This osmotic change may be effective in several 
ways. (a) It may be that the response is due to a change in 
the osmotic pressure of the solution in general. When the saline 
constituents of the solution are increased or decreased as 4 
whole there is a corresponding change in the so-called osmotic 
strength. (4) However, it may be, as suggested by Copeland's 
recent work,’ that the plant is more influenced by osmotic pres- 
sure when this is derived from one salt than by the same pressure 
_ derived from another. And if this be the case, then this influ- 
ence is as complicated a one as that of the change in the com 
ditions enumerated under (1} above. (3) Finally, the response 
in the plant may be due both to the stimuli from chemical com- 
position and to those from osmotic pressure, a combination of 
(1) and (2) above. 
My experiments were devised to determine primarily whee 
the stimulus is a chemical or a physical one. For this it § 
necessary to have solutions in which the relative and absolute 
amounts of saline constituents can be varied without changing 
the osmotic pressure of the salt content as a whole, and seed 
must be brought about, as far as may be, without the saclay 
tion of any new conditions. It is necessary first to know Fe 
Osmotic pressure of the complete Knop’s solution. Now 
P : is € ual, 
pressure of any weak solution of several constituents 6 2 . 
essures 0! 
2 , we salts 
the constituent salts, as these pressures would exist if eee 
were ; i lution whose V 
Separately put into a simple so makes it # 
equaled that of the complex one. This principle 
simple matter to calculate the pressure of complex 
Gad. 24: 39% 
SCoPELAND, E. B.: The relation of nutrient salts to turgor. BOT 
76 
