i7, 6 Trelease: Salt Requirements of Wheat Plants 601 
to interpret the experimental results furnished by this study. 
The growth and development of a particular set of culture plants 
is, of course, controlled by the environment, which comprises a 
large number of different kinds of effective conditions. An 
attempt was made in these experiments to' control only a small 
number of the conditions that may influence the plant. Of these 
effective conditions of the environment, only the initial concen- 
tration (osmotic value), the initial salt content of the solution, 
the frequency of renewal of the latter, and the duration of the 
experiment were controlled and taken into account. Other con- 
ditions that might act upon the plants and might alter the 
influence of the controlled conditions were allowed to vary as 
they did in the experiment greenhouse, without being taken into 
account at all. Among such uncontrolled conditions may be 
mentioned, for illustration, the temperature of the nutrient solu- 
tion and of the air around the foliage, the composition of this 
air, the radiation, etc. 
Since it is apparent that no solution remains unaltered after 
the introduction of the plants, the frequency of renewal of the 
nutrient solution must be considered a very important environ- 
mental condition. It is well known that the absolute plant 
values may be greatly changed by altering the frequency of re- 
newal of the solution, and with any other frequency of renewal 
than the one employed in these experiments the comparative 
results might have been very different from those recorded in this 
paper. Other changes in ,the comparative plant values might 
have been induced by aerating the culture solutions. The tem- 
perature of the culture solution also must have an important 
effect upon the comparative plant values. With other solution 
temperatures than those employed, the comparative values 
obtained might have been very different from those that were 
obtained in these studies. 
It appears probable that any given set of solution conditions 
may produce very different effects upon the plant according to 
the kinds and intensities of the conditions other than the solution 
that are effective. If two duplicate series of different sets of salt 
proportions or of different total concentrations of the nutrient 
solution were carried out, one with higher air temperature or 
with more sunshine than the other, it might well happen that the 
comparative plant values obtained from one series might be very 
different from those obtained from the other. It seems possible, 
for example, that lower optimal concentrations might have been 
obtained in series IV under aerial conditions that favored higher 
