ECOLOGY OF WOODLAND PLANTS. 371 
Fig. 16 shows the section of a leaf from a plant growing in 
a moist situation under the shade of Beeches. It is exceed- 
ingly thin, the epidermal cells are very thin-walled and contain 
numerous chlorophyll granules, while the mesophyll is reduced 
to two or three cells in thickness. 
All these variations in structure may be found within a very 
limited area; e. g., the woods at Armitage Bridge, as they afford 
all the necessary conditions. These woods skirt the edge of the 
Gritstone Plateau and extend over the steep slopes. Their 
higher parts are dry and sandy, and the soil is covered with a 
shallow peat, and the plants are exposed to the adverse conditions 
of the plateau generally. Bracken occurs in patches among the 
ericaceous undergrowth, and in the less protected parts develops 
extreme xerophytic characters. Over the moister, more sheltered 
slopes, protected by the Oak, it becomes more mesophytic in 
structure ; while in the deep shade area in the slope, under Beech 
and Elm, it becomes extremely attenuated and eventually dies 
out. 
The amount of shade produced by a given species of tree is not 
always the same; consequently we do not necessarily find shade- 
struetures developed in herbaceous plants under shade species. 
Closeness of planting, age of tree, and the condition of its 
growth have to be considered. 
Pteris aquilina. | Leaf-stalk. 
The examination of leaves from these different habitats 
showed that not only was the leaf-blade modified in structure 
according to environment, but also the leaf-stalk. For purposes 
of comparison, transverse sections were made in each case 14 in. 
below the lowest pair of leaflets. These are shown in outline in 
figs. 17, 19, 21, 23, 25, 27 (p. 372). Portions of these are shown 
x 150 diam., taken through the tissues of the same region in 
each ease. Fig. 17 is a section from a plant growing in a dry 
windy situation in an open Birch wood on the Gritstone Plateau. 
Here we see (fig. 18) the stereom is very strongly developed, and 
the cavities of the fibres are reduced to minute pores. This band 
is 13 or 14 cells deep, encroaching closely on the outer steles, 
and the thiekening is continued into the ground-tissue still 
further. If portions of these leaf-stalks are cut off and tapped 
together, they ring like dry bones. The stereom as here developed 
