Results of Water Culture Experiments . 431 
respiration. It is possible that in this accumulation of carbon dioxide in 
the culture solution is to be found the explanation of the less growth 
of plants growing in solutions which are not renewed, but which never- 
theless contain enough salts for a more rapid growth. The following 
numbers, taken from a previous paper of the writer’s, exhibit this point : 
Initial concentration 
of food solution . 
Solutions 
renewed. 
Relative total quantity 
of salts supplied. 
Growth. 
Time of growth. 
1*0 
Once 
2*0 
0*2 
Eight times 
i-8 
0*05 
Eight times 
o-45 
0*31 April 30 — June 9. 
©•62 April 28- — June 6. 
o*47 April 28 — June 6. 
A diminution of the oxygen dissolved in the culture solution might 
also affect growth and act as a limiting factor. In the absence of any data 
it is impossible to say how far this factor may be operative. The partial 
pressure of each gas in the culture solution will always be tending to come 
into equilibrium with the partial pressure of the same gas in the atmosphere, 
but there is no doubt a constant lag in the bringing about of this 
equilibrium. 
3. Concentration of the Nutrient Solution. The extent to which con- 
centration of the nutrient solution can act as a limiting factor is difficult to 
examine, because from the moment the plant roots are put in the solution 
the concentration will be altering. It was in an attempt to minimize this 
difficulty that the writer changed the water culture solutions at frequent 
intervals. Even then the concentration of the nutrient solution will only 
remain very approximately constant when the rate of growth is slow. 
This was the case with the writer’s experiments, in which it was shown that 
the variation in the concentration of the nutrient solution over a fairly wide 
range has little influence on the dry matter produced. In the lowest 
strengths of solution employed there was a slightly less rate of growth, but 
this might have been due to the insufficiency of one of the nutritive salts 
during the later days of growth. 
Dr. Brenchley states in her recent paper that when plants ‘are grown 
in water cultures under favourable conditions, the concentration of the 
nutrient solution, up to a comparatively high strength, has a great effect 
upon the rate of growth ’. As a matter of fact her experiments do not 
support this statement. Indeed her results properly examined rather 
indicate the reverse. In the following table, compiled from Dr. Brenchley’ s 
results, it is shown how much better growth results in solutions of low con- 
centration when these are frequently changed, than when they are not. 
The improvement in the case of higher concentrations is relatively much 
less. In order to show this clearly the weight of the plant in the never- 
changed solution is taken as unity in the case of each concentration in each 
series, and the weights of the corresponding plant in the once changed and 
frequently changed solutions are given relatively to this. 
