346 The Philippine Journal of Science ms 
If there were two limiting factors, then the intensity of both 
would have to be increased to augment the rate of the process, 
as increasing only one would still leave the other as a limiting 
factor. It would follow from this that it could not be possible 
under any condition to increase the intensity of either of two 
factors independently and get an increase in the rate of a pro- 
cess in both cases; for, if either were limiting, an increase in 
the intensity of the other would be without effect. On the other 
hand, if neither factor were limiting, then some other would be, 
and an increase in the intensity of either or both of the two 
factors that are not limiting would be entirely without effect. 
This point of view is clearly shown in Blackman’s discussion 
in the paper cited above, and also in a paper on assimilation 
in submerged water plants.® 
In the latter paper Blackman and Smith determine the rela- 
tion of variations in the concentration of carbon dioxide to 
photosynthesis ; also of two different temperatures and two light 
intensities to photosynthesis. They presented these results in 
the form of a diagram on page 402, in which the ordinates 
represent the assimilation. Separate curves were drawn re- 
presenting the relation to assimilation of the carbon-dioxide 
supply, the temperature, and the illumination. Concerning these 
curves they write as follows: 
* * * We can by inspection tell exactly what minimum of C0 2 , 
temperature, and light will be required for any one of the values of 
assimilation on the ordinates by following the horizontal line right across 
the diagram and noting where it intersects the three curves respectively. 
Reciprocally, given any hypothetical combination of different magnitudes 
of the three factors, we have only to note in the diagram the values of 
assimilation corresponding to each of them, and we can foretell that the 
value of assimilation actually attained in that combination will be the 
lowest of the three values. 
Now, according to this last quotation, if temperature were 
the limiting factor, an increase in carbon-dioxide supply or illu- 
mination would appear to have no effect. Blackman and 
Smith’s paper was supposed to demonstrate the action of the 
theory of limiting factors as proposed by Blackman. 5 6 It may, 
therefore, be interesting to examine some of the results. 
The first section of the paper deals with the relation of the 
5 Blackman, F. F., and Smith, A. M., Experimental researches on 
vegetable assimilation and respiration. IX. — On assimilation in submerged 
water-plants, and its relation to the concentration of carbon dioxide and 
other factors, Proc. Roy. Soc. London B83 (1910-1911) 389-412. 
6 Blackman, F. F., and Smith, A. M., op. cit. 
