on the Growth of Barley and Wheat in Water Cultures. 85 
and in some cases (Series 1 and 3) the approximation is fairly close. This 
suggests the possibility that more frequent renewal of the solutions, main- 
taining more evenly the balance of the nutrient salts, would be followed by 
as much growth in the N/5 as in the N concentration, although the tendency 
is for growth to fall behind in the lower strength with small provocation. In 
other words, if it were possible to arrange an experiment in which the 
balance of the nutrient solution was kept constant by the automatic 
replacement of the food-salts absorbed, it is conceivable that plants in these 
two concentrations might produce equal quantities of dry matter. But 
there are indications that toxic effects would set in under these circumstances 
in the N solution, as some of the constituents might be present in so great 
a quantity as actually to put a brake on plant growth. In the N/5 solution, 
on the other hand, the probability of such action is considerably less, and 
the plants would continue to make full use of the food-salts and would 
approximate in growth to those in the N solution. If this supposition 
be correct it is not true to say that the plant is indifferent to the variation in 
the strength of these two solutions, but that it responds to increased 
strength by increased growth. With the highest concentration, N, however, 
another factor, that of toxic action, comes into play, counterbalancing the 
increased growth and reducing it to the level of that attained with the lower 
(N/5) concentration. Further experiments are being made to obtain more 
definite information on this point, and also to find out whether there is 
an optimum concentration for growth or whether the plant will grow equally 
well within a certain range of the higher concentrations. The main point 
at issue at present is not that of equal growth in varying concentrations, but 
that of the great dilution at which it is claimed that such equal growth can 
be made. With concentrations below N/5 a very different result is obtained. 
The more frequently the solutions are changed, i. e. the closer the con- 
centrations approach to a state of constant balance, the more marked is the 
drop in the dry weights as the strength of the solution decreases. This 
implies that with the lower strengths the plant is living in a condition of 
semi-starvation. When the solutions are changed frequently, the improve- 
ment of growth is the more marked the higher the concentration (up 
to a certain limit, N/5 in this experiment), owing to the sum total of food 
supplied approximating more closely to the needs of the plant for optimum 
growth. It is difficult to imagine that, even if a constant balance of food- 
salts could be maintained, the plants in the solutions below N/5 would in 
any way approach those in the higher strengths, as, if this were indeed the 
truth, some indications of it would have been obtained by some incipient 
approximation of dry weights, such as occurred with plants in N and 
N/5 solutions, instead of a marked and decreasing divergence of these 
weights as the concentrations rose towards N/5, when solutions were fre- 
quently renewed, As a matter of fact, the actual dry weights of the N/10 
