OF THE BRACKEN IN RELATION TO ENVIRONMENT. 665 
under glass bell-jars and under identical conditions of illumina- 
tion; but one plant was supplied with normal air, the second with 
very dry air, and the third with very damp air. These experiments 
gave the very interesting result that in damp air and iu dry air 
the leaves showed a marked structural divergence from those in 
normal air, and of the type characteristic of shade-leaves for 
those in damp air, and of sun-leaves for those in dry air. 
Unfortunately, the author makes no statements regarding the 
structure of the leaf grown under glass as compared with leaves 
of the same plant grown in the open. 
Though it is just possible that, to a certain extent, sunlight 
(apart from its action in increasing chlorovaporisation) may act 
as a stimulus, which in some unknown way helps to determine 
the mature structure of the leaf, yet it seems probable that the 
chief determinant (which must act during the later stages of de- 
velopment of the leat) is a factor which would be represented by 
the sum of the external factors inducing loss of water from the 
surface of the leaf, coordinated in some mathematical relation 
with the sum of the factors (external and internal) regulating the 
supply of water to the leaf *. 
The different factors, with which one is concerned in the study 
of ecology T, need not be specially discussed here; but it may be 
pointed out that in natural habitats it is extremely difficult 
to determine the preponderance of any one factor. For in 
localities exposed to sunlight (which increases chlorovaporisation) 
there would usually be several other factors working in the same 
direction, either by increasing evaporation or by reducing water- 
supply, e.g., one or more of the following :—dryness of air, 
exposure to wind, dryness or coldness of soil. In the greenhouse 
experiment, though the high temperature has to be taken into 
account, it would appear that dampness of the atmosphere and 
protection from wind may have been stronger factors in one 
direction than fairly strong illumination is in the other direction ; 
or, more probably, it is simply that in this case the balance of 
factors was such that, the absorptive power of the roots being 
increased by the high temperature, and transpiration being 
kept fairly low by the dampness of the atmosphere, there was no 
* Of. Pfeffer, ‘The Physiology of Plants,’ Engl. ed. p. 239, m 
+ Enumerated by Warming and others ; Warming, ‘ Oekologische Pflanzen- 
geographie’ (tr. Knoblauch). 
