EXPOSURE ON LENGTH AND BREADTH OF LEAVES. 383 
were shortest, the sand next, then the lime, then the manure, and 
the leaf-mould plants were the highest. Almost exactly the 
same order was observed in the amount of branching per plant, 
in the number of flowers per plant, and in the average horizontal 
space and depth to which the roots extended. A full account 
will be found in the ‘ Gardener’s Chronicle’ (Dec. 6, 1890, and 
the following number). 
The result is perhaps explained by the fact that, excepting 
peat, which is a positively injurious soil, the amount of water in 
the soil is probably most in leaf-mould and manure, less in lime, 
and less again in sand. Hence transpiration will be most active 
in the case of leaf-mould, less in manure, still more so in lime, 
and least in peat and sand. 
It is doubtful, however, whether this conclusion is warranted. 
Still, even if we exclude the case of these experimental beds as 
indecisive, we can, I think, consider that the main conclusion I 
have drawn is pretty safe. Sorauer’s, Dufour’s, Wiesner’s, and 
my own measurements show that plants grown in sheltered 
situations have more drawn-out leaves, just as they have longer 
and thinner stems. 
This is an important point in systematic character, as a lan- 
ceolate, linear, or ovate leaf simply arises from the different pro- 
portions between length and breadth. If, then, climate or 
exposure can, as I think I have shown, produce variations in this 
respect on which natural selection may afterwards begin to act, 
we can see how a new species may be formed. 
The difference between Ranunculus reptans and R. Flammula, 
for instance, is chiefly a difference in the leaf-ratio, the leaves 
are longer and narrower in the second form; and Ross has ex- 
perimentally changed the one into the other by growing them 
under different conditions, and, so far as I can gather from his 
paper, his method consisted in inereasing the amount of moisture. 
The eause of this change is not explicable without a somewhat 
rash and speculative expression of views. There is, however, no 
doubt that the effect of exposure or accelerated transpiration is 
to increase the toughness and rigidity of the leaf and especially 
to thicken the epidermis. This follows from the culture experi- 
ments of Costantin, Dufour, Sorauer, Wiesner, &c. The com- 
parative researches of Von Höhnel, Klausch, Schenck, Noack, 
Wiesner, ané Grevillius show that in nature a greater toughness 
is produced in the same species by greater exposure. 
