Blackman. — Optima and Limiting Factors. 291 
suppose the light falling on the leaf to be sufficient for the decomposition 
of 10 c.c. of carbon dioxide per hour, then twice the external pressure of 
carbon dioxide will be required to reach the limit and the angle of 
the curve, which will now be ABDE. With still stronger light we 
should get ABDFG . Those who would be prepared to admit that 
a curve like ABC shows an optimum, only with a very long drawn-out 
top, would have to further admit that for each intensity of light falling 
on a leaf there is a different optimum amount of carbon dioxide. This is 
not to be entertained. 
The light-energy available fixes an upper limit to the carbon dioxide 
that can be decomposed, and when that amount is attained, which even for 
direct sunlight could be provided with a current of air containing less than 
1 per cent, if the current were sufficiently fast, the limit of effect of carbon 
dioxide is reached : any more provided is wasted, and has no further effect 
till many times that concentration is reached and a general depressing 
effect comes in. Just as little can one speak of an optimum amount of 
carbon dioxide required to use up a fixed amount of radiant energy (i.e. 
a given intensity of light) as one can speak seriously of the ‘optimum 
amount ’ of water required to fill a litre flask, while to attempt to speak of 
an optimal amount of carbon dioxide for assimilation in general is like 
speaking of 550 c.c. as the optimal amount of water to fill fiashs, when the 
two flasks in question happen to be the one a litre flask and the other 
a 100 c.c. flask. 
With regard to the function of respiration, a factor that sometimes 
comes markedly into play as a limiting factor is the amount of plastic 
material available for oxidation, and it is by no means easy to separate 
analytically the effects of this factor and those of the temperature factor. 
