On the Interpretation of the Results of 
Water Culture Experiments. 
BY 
WALTER. STILES. 
Introduction. 
W HEREVER a process is a function of several independent factors, it 
is necessary to consider the possible influence of all the others 
when the relation between the rate of the process and any one of the 
factors is examined. In plant physiology the outstanding example of this 
is to be found in carbon assimilation. The processes summed up under 
this term depend upon at least three independent variables : carbon 
dioxide, temperature, and illumination. As a result of F. F. Blackman’s 
exposition, it was shown that increase in any one of these factors brought 
about a corresponding increase in assimilation, provided the other factors 
were present in quantity sufficient for the assimilation to proceed at the 
increased rate. Thus to choose one factor, temperature, the rate of the 
assimilation is more than doubled if the temperature is increased io° C., 
but it is self-evident that when the assimilation proceeds at this increased 
rate more than twice as much carbon dioxide will be used up in any given 
time than at the lower temperature. Now if in this time the leaf is not 
supplied with this larger quantity of carbon dioxide, the assimilation 
obviously cannot proceed at the maximum possible for that temperature. 
The amount of carbon dioxide under these conditions is a limiting factor. 
In water culture experiments there is usually measured the increase of 
growth, during a certain time, corresponding to some definite composition 
of the nutrient solution. In other words, experiments with water cultures 
attempt to determine the effect upon growth of factors acting through the 
root system. As growth depends upon so many factors, it is very necessary 
in drawing conclusions from water culture experiments to bear in mind the 
possible action of factors other than the one under investigation. That 
the neglect of this is probably the cause of divergent results obtained by 
different experimenters with the water culture method will be shown in 
this paper. 
The Complexity of the System. 
Growth, in the sense of increase in the total dry matter of a single 
plant, is the resultant of a large number of inter-related processes, of which 
[Annals of Botany, Vol. XXX, No. CXIX. July, 1916.] 
