Aspects of Ecology. 
236 
There is further another class of disturbing factors not taken 
account of in Dr. Clements’ simple scheme of correlations, a class 
that may be called limiting factors. For instance the statement 
that photosynthesis in Nature is proportional to light intensity is 
only true up to a limit set by the amount of C0 2 that can reach the 
plastids by diffusion. With greater intensities of light beyond that 
limit there can be no further increase of photosynthetic work and 
the light energy absorbed is deflected to some other work. It is of 
the first importance to note that the available C0 2 becomes a 
limiting factor to assimilation within the middle range of normal 
illumination (at somewhere about l of full summer noon-tide 
illumination). This limitation is therefore daily at work and it 
results that the proportionality between “ light stimulus ” and 
“photosynthetic response” will be in practice sharply arrested at 
soms arbitrary point depending on the structure of the leaf. 
Then, again, as regards transpiration we have also a natural 
arbirtary limitation to the (inverse) proportionality which it funda¬ 
mentally bears to the humidity of the air. As long as the stomatal 
openings remain of the same size there is a proportionality between 
dryness and stomatal transpiration. But at a certain dryness of 
surrounding air—individual to the plant—the stomata close and put 
an end to the proportionally of factor and response as an actual 
general working relation in the habitat. 
Facility of conduction of water must too be sometimes a limiting 
factor in determining whether a drier habitat produces increased 
absorbing root system or decreased transpiring leaf system. 
The fifth principle is a hypothesis to explain the fact that many 
bog plants are notoriously xerophytic in structure and that many 
sun plants transplanted to a much diminished light show no 
structural change. This conception of varying plasticity of species 
causes the reader some uneasiness in its present purely hypothetical 
form as it would be so easy to apply it in apparent explanation of 
all exceptions to the expected correspondence between factor and 
functional and structural response. 
In fairness to Dr. Clements it must be stated that he does not 
regard all these principles as incontrovertible. He says at the end 
of them “the unimpeachable facts are relatively few in this domain 
and their present correlation slight. In the treatment which follows 
the method of multiple working hypotheses (our Italics) is uniformly 
employed. No apology is necessary since the whole endeavour is to 
indicate the proper point of attack.” 
