ECOLOGY OF WOODLAND PLANTS. 397 
Deschampsia), owing to soil-conditions, oceupy the same layer; 
and after we have made allowance for differences in food require- 
ments, they still form a competitive association, and sometimes 
one, sometimes the other species gains the upper hand. In this 
connection very little work seems to have been done, aud further 
study will doubtless lead to interesting results. 
In studying the vegetation in the transition region from the 
relatively dry Millstone Grits to the moister Coal-Measures, we 
find that the line of demarcation, though evident, is not sharply 
drawn between the two biological types; but as the xerophytes 
invade the region of the mesophytes, and come under the 
mellowing influences of moisture and shade, increased tempe- 
rature and greater protection, they tend to lose their xerophytie 
characters and take on mesophytie characters. The reverse also 
holds good—that as the mesophytes eneroach on the xerophytes, 
and come under the influences of drier and more rigorous con- 
ditions, they develop xerophytie characters. 
These changes in soil and other conditions act more com- 
pletely as barriers to some species than to others, but in the 
case of species not so restricted in their distribution, differences 
in physiological water and food-supply, presence or absence of 
peat or humus, produce striking modifications in their form and 
structure (e.g. Pteris and Vaccinium Myrtillus). 
The influence of the dominant tree determines in a varying 
degree the distribution of the species (e. g. Pteris), and not only 
affects the amount of transpiration in the plants of the under- 
growth, but also brings about modifications in structure, resulting 
from the amount of shade produced and accompanying conditions, 
e.g. Pteris, Scilla festalis, Deschampsia flexuosa, Holcus mollis, 
Heracleum Sphondylium, Lamium Galeobdolon, and Mercurialis 
perennis—each showing well-marked sun and shade, xerophytic 
and mesophytie structures according to environment. 
Variations in light-intensity or the direction of the incident 
rays not only affect the structure, but also the habit of the plant, 
e. g. Holcus mollis and Vaccinium Myrtillus. 
The several species of an association vary considerably as to 
their power of adaptability, and therefore in their range of dis- 
tribution, e. g. Pteris as compared with Calluna. In general the 
less plastic a species is, the narrower is its range of structural 
variation and the more restricted is its distribution. 
LINN. JOURN. —BOTANY, VOL. XXXVII. 2r 
