ON THE RELATIVE LENGTH AND BREADTH OF LEAVES. 375 
The Effect of Exposure on the Relatiyé Length and Breadth 
of Leaves. By Q. F. Scorr Erriór, M.A., B.Se., F.L.S. 
[Read 18th December, 1890.] 
Tue effect of sunlight on the absolute size of leaves is known, 
from the magnificent researches of Stahl, to consist chiefly in a 
reduction of the leaf-surface. The works of Volkens, Tschirch, 
Schenck, and others have proved that this reduction of the surface 
is a common characteristic of those plants which are more par- 
ticularly exposed to strong sunlight, e. g. desert plants and plants 
living on dry sea-sand. 
It is also well known that along with this reduction in surface 
there is usually an increase in thickness of the leaf—that is to 
say, in the same species when grown under specially exposed 
conditions. 
No one who has read the evidence on the subject ean, I think, 
doubt that this tendency to reduce the amount of leaf-surface, 
and at the same time increase its thickness, is connected with 
the greater transpiration in more exposed habitats, and that the 
effect of sunlight is (at least to a great extent) indirect and 
produced by its favouring transpiration. 
It is also generally recognized that shade-plants tend to have 
longer internodes than those which are exposed to sun and 
wind (Wiesner, e. g., has shown that the dandelion, when grown 
in moist still air, forms internodes); and this “drawn” cha- 
racter of plants grown in shady or sheltered spots is quite 
familiar to open-air botanists, and one can scarcely doubt that it 
also is connected with the transpiration being less in such places 
than it is in exposed ground. 
If one assumes these facts, viz. that exposure in a general 
sense (to wind and sun) contracts the internodes and produces 
a diminution in the absolute length and breadth of the leaves as 
well as a transverse thickening, then one would expect that the 
relative length and breadth would also be changed, so that in 
exposed plants the leaves would, in most cases, become broader 
proportionately. There is some reason for the above assump- 
tions, as I do not think any one who has collected plants in hot 
countries can have failed to notice these tendencies, and, more- 
over, the literature of the subject is very large and remarkably 
harmonious. 
LINN. JOURN.—BOTANY, VOL. XXVIII. 21 
