134 Ingvar Jorgensen and Walter Stiles. 
temperatures is measured over a considerable time it is certain 
that a temperature will be found at which assimilation is at a 
maximum. This apparent optimum temperature will however vary 
according to the time elapsing between the commencement of 
assimilation at that temperature and the actual measurement of 
assimilation owing to the rapid falling off in assimilation due to the 
time factor. 
We shall later discuss the time factor in more detail, but in 
this place we would comment on the use of such expressions as 
‘ time factor.’ It is of course desirable that the same terminology 
should be used wherever possible in physiology as is employed in 
pure chemistry and physics. But while analysis of physiological 
processes has not proceeded far this is not always possible, and it 
is to the credit of F. F. Blackman that he has introduced the terms 
‘ limiting factor,’ ‘ time factor,’ which permit discussion of 
physiological processes without involving premature assumptions 
as to their nature. 
The work of F. F. Blackman and his pupils has been largely 
concerned with the influence of the various factors temperature, 
light and carbon dioxide supply on the rate of carbon assimilation. 
Before the publication of his ‘ Experimental Researches on Vegetable 
Assimilation and Respiration ’ there had indeed been much 
work on the influence of these factors on assimilation, but as none 
of these previous workers had recognised the principle of limiting 
factors it seems unnecessary for us to discuss their results here. 
Again the search for an optimum value of the various factors has 
not helped to elucidate the problems. 
We shall first deal with the work of Blackman and his pupils 
on the relation between assimilation and the chief environmental 
factors, carbon dioxide supply, light intensity and temperature. 
Blackman expresses this relation as being such that “ the magnitude 
of this function in every combination of these factors is determined 
by one or other of them acting as a limiting factor. The identification 
of the particular limiting factor in any definite case is carried out 
by applying experimentally the following general principle. When 
the magnitude of a function is limited by one of a set of possible 
factors, increase of that factor, and of that one alone, will be found 
to bring about an increase of the magnitude of the function.” 
We give here (Fig. 6) the curves obtained by Blackman and 
Smith (1911 b) showing the inter-relationship between carbon 
assimilation and the three external factors in the case of Elodea. 
