412 On Vegetable Assimilation and Respiration. 
fig. 7, by which it is possible to foretell what value of assimilation in Elodea 
will be attained in any combination of medium magnitudes of the three 
factors of the environment. In this diagram, against the different values of 
assimilation as ordinates, are ranged three separate curves showing the 
degrees of CO2-supply, temperature and illumination, which are respectively 
essential for the attainment of each value of assimilation. For any hypo- 
thetical combination of the factors, it follows, by the principle of limiting 
factors, that if the three functional values corresponding potentially to these 
be ascertained from the diagram, then the actual magnitude of assimilation 
attained with that combination of factors will always be the smallest of the 
three potential values. 
8. The experiments in this paper deal with such moderate intensities of 
assimilation as may be fairly well maintained for several successive hours. 
With more intense assimilation the values soon fall off by the action of 
internal factors grouped at present as the é¢me-factor. Experiments in which 
this additional factor has to be reckoned with will be considered in a later 
paper. 
9. Section IV contains a critical account of the work of previous 
investigators who interpreted their results on the assumption that there was 
a primary optimum in the relation between assimilation and each external 
factor. The substantial work of Pantanelli (1903) led him to the conclusion 
that the position of the optimum for any one factor shifts with the 
magnitude of the other concurrent factors. This can only be regarded as 
a transitional point of view and from this we have advanced to the con- 
clusion that the whole conception of optima in this connection is mapplicable 
and breaks down completely on careful analysis. It is shown in detail that 
all the experiments of previous workers are more harmoniously interpreted 
from the point of view of interacting limiting factors than by the conception 
of optima. 
