ECOLOGY OF WOODLAND PLANTS. 987 
the chlorophyllaeeous cells above and below. Towards the 
lower surface the cells are small, four- or five-sided in section, 
and for about four rows in depth distinctly thickened (fig. 54), 
forming a shield-like stereom-plate. Towards the upper surface 
the cells are thinner-walled and rather larger. 
Fig. 55 is a section through the base of the blade of the 
reflexed shade-form. In this we see that the blade has been 
bent over by the great elongation of the cells of the upper 
surface, many of which show distinct collenchymatous 
thickening. 
As Schimper points out (83), a plant strives in various ways to 
obtain an ecological optimum of light. Holcus mollis affords 
an interesting example of this. In sunny situations it assumes 
an erect sun position. In the somewhat diffuse light of an Oak 
wood its leaves curve in such a way as to expose their flat 
surfaces to light ; and when growing under trees where the light 
above is practically cut off, but strikes the plant obliquely or 
horizontally, its blades become strongly and permanently re- 
flexed, and their flat surfaces exposed to the direetion of the 
incident rays. 
Vaccinium MynriLLUs, Linn. Bilberry. 
This species is especially abundant in the open Pine woods * 
of the Millstone-Grit area, and in the dry Oak and Birch woods 
it forms, together with Calluna and Deschampsia, the dominant 
vegetation of the undergrowth. It quickly dies out over the 
Coal-Measure shales and clays, and under the deep shade of 
Sycamore, Elm, and Beech, being confined in this area mainly to 
the drier soils over sandstone. To the west, it extends beyond the 
limit of the woods, isabundant along the dry moor edges and slopes, 
butis displaced to a large extent by cotton-grass (Eriophorum vagi- 
natum) on the deep ill-drained peat, to reappear again in somewhat 
extensive masses on the hill-summits at 1600 feet and upwards. 
* During a recent tour in the Eastern and Swiss Alps, I was much struck 
by the habit of this species in the Spruce forests. Here it grows in forests of 
such density as appear to be fatal to it in the Huddersfield district, probably 
owing to the density of the smoke-cloud characteristic of this part of West 
Yorkshire; whereas the intense sunlight of the Alps penetrates the forests 
sufficiently to favour its development, but it assumes a very characteristic 
habit: it forms flat branched plates, all the leaves exposing their upper 
surfaces to the light, being a very striking shade-type. While the plants 
growing in the open have their branches more erect, the leaves are erect and 
parallel to the stems and so expose their edges to the light, a typical sun-habit, 
