Optima and Limiting Factors. 
BY 
F. F. BLACKMAN, M.A., D.Sc., 
Fellow of St. John' s College , Reader in Botany in the University of Cambridge. 
With two Diagrams in the Text. 
I N this article it is proposed to subject to critical consideration the 
conception of the ‘ optimum 5 as a primary general relation between 
physiological processes and the external or internal conditions which affect 
them. 
In treating physiological phenomena, assimilation, respiration, growth, 
and the like, which have a varying magnitude under varying external 
conditions of temperature, light, supply of materials, &c., it is customary to 
speak of three cardinal points, the minimal condition below which the 
phenomenon ceases altogether, the optimal condition at which it is exhibited 
to its highest observed degree, and the maximal condition above which it 
ceases again. 
As the maximum temperature for most metabolic processes is very 
meat* to the death point, exact location of it is attended with considerable 
experimental uncertainty and precise data are generally wanting. In 
practice, attention is usually concentrated upon the optimum of the 
condition and upon the general form of the middle part of the simple curve, 
which is usually accepted as a satisfactory graphic expression of the relation 
between the function and the condition. 
In the treatment of the assimilation of carbon dioxide in all text- 
books we find mention of optima of temperature, of light, and of carbon 
dioxide-supply for this process. After some years of experimental study 
of the effect of external conditions upon carbon-assimilation the writer 
has demonstrated that much of this treatment is quite incorrect, and 
from this position has passed to the general conviction that there is much 
that is misleading in that treatment of the effect of an external condition 
which involves giving definite values to its cardinal points. 
[Annals of Botany, Vol. XIX. No. LXXIV. April, 1905.] 
