PHYSIOLOGY OF THE LEAF. 
6'^ 
Size in different leaves ; in some, witli the most powerful mag» 
nifiers, the cells are scarcely perceptible; in others, they may 
be seen with the naked eye. Tliese cells are of important nse 
in the secretion and communication of substances through the 
leaf ; and may thus be considered as a kind of gland, having a 
communication with the vascular system. The covering of the 
leaf, or the cutide^^' guards the vascular and cellular system 
from injury, and is the medium by which the leaf performs the 
important functions of absorbing nourishment, and exhaling 
such substances as are useless, or hurtful. The cuticle is some- 
times covered with downy or hairy glands^ which seem to 
afford security against changes of weather ; such plants are 
capable of enduring a greater degree of heat than others. In 
some cases, the cuticle is covered with a transparent "carnish^ 
which preserves the plant from injury by too much moisture, 
and adds to the beauty of the leaves. The trees of Abyssinia 
and some other comitries, which are subject to long rains and 
continued moisture, are thus shielded from the injurious effects 
of the weather. 
"When the surface of the cellular tissue 
is more ample than the vascular net- 
work, the leaf is rugose (Fig. 62, A 
where, for every swelling of the upper ft 
surface of the leaf, there is a correspond- B 
ent depression of the under surface : the ^ 
sage has a leaf of this kind. When the 
net-work exists, but the meshes are destitute of cellular tissue, 
the leaf presents the appearance of lattice-work, and is said to 
be cancellated. 
The leaves of an aquatic plant of Madagascar {Hydrogeton fenestralis. Fig. 62, h)* 
are of this kind. Another example of this leaf is seen in the Claudea elegans (Fig. 
62, c), a species of marine Algfee, found in New Holland ; the veins are parallel to 
the sides, and cross the nerves. 
61. Bjpecial functions of Leo/ves. — Leaves expose the fluids of 
the plant to the influence of air and light ; they aid in the 
elaboration of the various vegetable secretions, in the forma- 
tion of wood, and in the absorption of fluids and gases. In the 
cells of the leaves changes take place by the agency of light, 
by which oxygen is given off, and carbon remains in a fixed 
state. Exhalation^ or transpiration^ is the process by which the 
superabundant juices of the plant are given off; it takes place 
* The cuticle is sometimes called epidermis, from epi, around, and derma, skin ; the true skin being 
not the outer covering, but a cellular substance beneath : thus, the thin skin upon the back of the 
hand, which so easily becomes rough, is the cuticle, or epidermis (sometimes called the scarf-skin), 
while the real skin is below. 
Cuticle — Important office of the leaf— What is a rugose leaf ? — What is a cancellated leaf?- -Er 
tlaiu Fig. tJ2.— 6L Functions of leaves. 
