92 
Ingvar Jorgensen and Walter Stiles. 
Blackman has brought out the important relation between 
environmental factors and carbon assimilation, and has formulated 
the principle of limiting factors in regard to their co-operation. 
However, absolute rules cannot be made as to amount of assimilation 
under any definite environment, owing to the complexity introduced 
by the existence of unknown internal factors. Willstatter has 
attempted to analyse the internal factors, and has brought proof 
that chlorophyll is not the only internal factor, though what other 
internal factors there are Willstatter’s work does not show. Future 
work will have to investigate the inter-relation between the internal 
factoi's as well as the co-operation between the internal and external 
factors. 
However, the internal factors operative at any moment are a 
product of hereditary factors and environmental factors. It seems 
likely that an application of the principles of genetics may prove 
helpful in the analysis of internal factors in assimilation, and this 
application may give a method for controlling some internal factors. 
The aim, at present, of investigations on carbon assimilation is 
to be able to tell the assimilatory power of a plant with a known his¬ 
tory as regards environmental and hereditary factors when it is placed 
in a known environment. Then it becomes of industrial importance to 
discover how environmental factors can be modified so as to give the 
maximum assimilation in relation to the inherited internal factors. 
We should like to emphasize that the popular idea that under 
natural conditions any particular factor, as for instance, light, is 
nearly always in excess, while some other factor, as for instance 
carbon dioxide, is nearly always limiting, is not justified. The power 
of the plant to utilise any environmental factor must undergo 
diurnal and seasonal variations depending on the interplay of the 
other factors. 
For instance the environmental factors, radiation and temper¬ 
ature, undergo daily and seasonal variations while although in regard 
to the carbon dioxide supply not much information is to be had, 
undoubtedly considerable variations occur (see, for example, Krantz, 
1909). In this connection may be mentioned the work of Kraus 
(1911) who successfully shows how great may be the variations in 
environment over a very small area. 
The importance of work on carbon assimilation depends not 
merely on its value in plant physiology and on its application in 
agriculture, but also, as we have emphasized in our introductory 
chapter, for the utilisation of radiant energy. For this reason in 
