OF THE BRACKEN IN RELATION TO ENVIRONMENT. 669 
extreme types, though some of the observations make this 
appear unlikely. Details of the outline of the pinne, on which 
character several varieties have been founded, were not specially 
considered. 
It may be added that the stomata are slightly raised, and, on 
an average, to about an equal extent in all the forms examined, 
including the greenhouse-leaves. The importance of this as a 
factor necessitating xerophytic leaf-structure cannot be estimated, 
because much exposed Jeaves are often more hairy on the lower 
side than less exposed ones. 
SUMMARY. 
In dry, exposed situations the leaves of the bracken are xero- 
phytic, and have a hypoderm, while in well-sheltered and shaded 
habitats the leaves are of the type of delicate “ shade-leaves,” 
having no hypoderm, and either weakly developed palisade or 
sometimes none. 
The same range of structure may be shown by the different 
leaves of the same plant or by different parts of the same leaf, 
when opposite external conditions are sufficiently localized. 
A plant, grown first in a damp greenhouse and then in the 
garden, produced shade-leaves in the former and sun-leaves in 
the latter. The greenhouse-leaves showed reduction of the 
indusia. 
The mature type of structure is not determined at an early 
stage in the growth of the leaf. 
The amount of illumination is probably not the only factor 
determining the structure of the leaf. 
The examination of material, collected from different localities, 
was carried out at the Jodrell Laboratory, Royal Botanic Gardens, 
Kew. My thanks are due to Dr. D. H. Scott, F.R.S., to whom 
I am indebted for several valuable suggestions. 
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