64 
PLAN1 MORPHOLOGY. 
sufficient importance to warrant consideration in this 
connection. 
III. THE LEAF. 
(a) SIMPLE LEAVES. 
The leaf is a lateral development on the stem and 
branches, in the axil of which a bud occurs. 
Leaves originate, like the branches, as the product of a 
meristem developed just beneath the epidermis. A 
simple leaf consists of a lamina or blade, which is usu- 
ally membranous and of a green color, and a petiole or 
stalk, which, however, may be wanting when the leaf 
is said to be sessile. Leaves may also possess a pair of 
leaf-like structures at the base, which are known as 
stipules. While the lamina of the leaf appears to assume 
a more or less horizontal position, it usually inclines at 
such an angle as to receive the greatest amount of dif- 
fused daylight. Wiesner has shown, for instance, that 
when plants are so situated that they receive direct 
sunlight only for a time in the morning, and diffused 
daylight during the rest of the day, the position of the 
upper surface is perpendicular to the incident ra} r s of 
daylight, and not to that of the rays of the morning 
sun. This phenomenon may be studied in the house 
geranium and other window plants. In endeavoring 
to explain this behavior of the leaves, Frank assumes 
it to be due to a kind of heliotropic irritability peculiar 
to dorsiventral 1 organs, and terms it transverse helio- 
tropism. 
The stem, as well as the petiole or stalk of the leaf, 
is also influenced by the light, and is said to manifest 
1 Dorsiventral is a term applied to any foliaceous part of the 
plant in which two surfaces are distinguished, each possessing a 
difference in structure, the one surface being known as the dorsal 
and corresponding to the lower surface, as in the leaf, and the 
Other as the ventral and corresponding to the upper surface. The 
