TEACHERS DEPARTMENT. 
IO9 
are smaller, and narrower in proportion to their length, than the 
shade leaves. Their margins are recurved, and the under surface 
is strongly concave. They assume the vertical position, while 
the shade leaves grow more horizontally. The sun leaves are in¬ 
variably the thicker. In those species which are pubescent or 
scaly on the lower surface, the hairs or scales are much thicker 
on sun leaves. The stomata, also, are more numerous. There 
is a more highly developed cutinized layer of upper epidermis in 
sun leaves; and a double layer of palisade cells, with but a single 
layer in the shade leaves; moreover, the intercellular spaces in the 
upper mesophyll are less extensive in the sun leaves. 
To ascertain the relative amounts of transpiration in the two 
kinds of leaves, the freshly cut ends of leafy twigs were imrfiersed 
in water contained in small test-tubes. Each stem, coated with 
wax to prevent evaporation, was sealed into' its tube, air being 
admitted through a capillary tube alongside the stem. Observa¬ 
tions were made out of doors, on sunny days, at a temperature of 
about 21 0 C., and a relative humidity of 67 per cent. 
With each kind of leaf under normal conditions, the sun leaves 
were found to transpire considerably more than the shade leaves 
(from 3 to 10 times as much), and the same general result was 
obtained when shade leaves were placed in full sunlight, or sun 
leaves in the shade. 
It is suggested that the sun leaves, being the more vigorous, are 
better able to transpire large quantities of water, and that their 
greater thickness gives them more interior evaporating surface, 
the xerophytic structure existing only for use in emergencies to 
protect the plant from injurious loss of water. 
These results are especially interesting because they are con¬ 
trary to the generally accepted theory that xerophytic leaf struc¬ 
ture precludes abundant transpiration. 
M. M. B. 
