ECOLOGY OF WOODLAND PLANTS. 369 
it is in the open, more exposed parts of this area that xerophytie 
characters reach their greatest development. In the treeless 
hill-region to the west it is a characteristic plant of the steep 
hill-slopes, where it is a striking feature in the landscape. These 
slopes offer it considerable protection, but the species dies out 
above as the exposure increases. The leaves here do not exhibit 
as a whole such strongly marked xerophytic characters as those 
of the Gritstone Plateau, for, although the plants are exposed 
throughout a considerable period to intense sunlight, the 
soil-conditions as regards moisture are on the whole more 
favourable. 
Growing in such diverse habitats and under such varying 
conditions, the plant shows extreme modifications in structure. 
Very numerous specimens have been collected and examined, 
and my observations on the structure of the leaf bear out fully 
those of Boodle (9), and his suggestion that “light is not the 
all-important factor determining the structure of the sun and 
shade leaves,” is supported by my observations of plants in 
natural habitats in this area. I should, however, say that light 
is only one of the factors, the other chief ones being wind and 
available moisture; for we find extreme shade-structures in 
sheltered moist situations under the shade of Beech and Elm, 
and the maximum sun-form in bright, illuminated, windy, dry 
situations on the Gritstone Plateau. 
The admirable pioneer work of Stahl (92), also of Haberlandt 
(42) and Schimper (83), laid the foundations for observations of 
this kind, and they have been since extended in many directions 
by numerous observers. For a more extensive bibliography on 
leaf-struetures and functions, reference may be made to 
Karsten (50 4) and Burgerstein (10). 
Figs. 13-16 (p. 870) show transverse sections of corresponding 
pinne from plants growing under different conditions. 
Fig. 13 is a section of a leaf from an open Birch wood on the 
Gritstone Plateau. Here the fronds are from 12 to 18 inches 
in height, are dark green in colour, and leathery, the plants 
barely overtopping the plants of Ling among whieh they grow. 
The epidermis has a thick cuticle, the cells of which are devoid 
of chlorophyll; beneath this is a nearly continuous hypoderm 
consisting of thick-walled colourless cells forming an aqueous 
layer. Between these are occasionally thin-walled cells which 
contain few or no chlorophyll granules. Below this is a well- 
