Influence of Aeration of Nutrient Solution. 187 
5. Climatic factors. Those factors influencing the growth of 
the plant which act through the aerial part, and which we may 
conveniently group under the term ‘ climatic factors,’ are of great 
importance in determining the activity of the plant, yet they are 
never taken account of in water culture experiments. They are 
principally temperature, light and carbon dioxide supply, on which 
assimilation is highly dependent. The effect of all these factors on 
influencing assimilation and consequently growth is well known, and 
their complex mode of interaction has been emphasized. As a 
simple example of how neglect of them may vitiate conclusions 
drawn from water culture experiments we may refer to the different 
results which might be obtained in winter and summer. Plants 
growing under two different sets of conditions at the higher 
temperatures and illuminations of summer might show different 
rates of growth on account of those differences in environment, 
whereas at the lower temperature and under the diminished 
illumination of winter, no differences in growth in the two cases 
might result, although those differences in environment maintained 
in the summer experiments were maintained also in the winter 
ones. This would be due simply to the fact that the temperature 
and light were limiting growth during the winter period, and if it 
were concluded that the differences in the other factors of 
environment produced no effect the conclusion would be false. 
As well as the factors of temperature and light, the carbon 
dioxide supply must be taken into consideration. This is not nearly 
so constant a factor as is generally supposed ; it makes a consider¬ 
able difference whether the air is stagnant or kept in motion. 
Movement in the air, by breaking down the diffusion shells which 
are otherwise produced over the stomata, is equivalent to an 
increase in the concentration of the carbon dioxide. 
A factor depending on these is that of spacing. If plants are 
growing closely together there may be competition between them 
for temperature, light and carbon dioxide, the term competition 
being used, of course, in its biological sense. Thus in the work of 
many experimenters in America, a number of plants are grown 
together in one culture jar. Recent work at Rothamsted has 
shown that when two plants are grown together in one culture 
vessel, a decrease in growth of the individual plant results, which is 
to be ascribed to competition between the aerial parts (3). 
6. Increase in Dry Weight as a Criterion of the Activity of the 
Plant. As we have emphasized in the general introduction to this 
paper, the measurement of dry weight affords no indication of the 
