LEAVES. 
43 
the axils of leaves. One leaf only arises from the same organic 
point ; a tuft fascicle of leaves is composed of the leaves ot 
an axillary branch so short that the bases of the leaves are in 
contact ; that is, the foliage develops without any elongation 
of the axis or the internodes. The complete leaf consists of 
the lamina iplade or limh)^ its petiole and pair of stipules at its 
base. Sometimes there is little expansion of this organ, bnt it 
is stalh-liJce j — generally the leaf is expanded horizontally, so 
as to present the under surface to the ground, the upper to the 
sky. On examining the cellular texture with a microscope, it 
is found that there is in each leaf an upper and an under 
stratum of differently arranged cells ; the upper stratum has its 
cells compact, and so arranged as to be least alfected by the di- 
rect rays of the sun, while the under stratum has a cellular 
arragement more favorable for evaporation or exhalation. 
50. The leaf is an expansion of the fibers of the bark, con- 
nected by cellular tissue^ developed in a symmetrical manner, 
as lateral appendages to the stem, and having a connection with 
its internal part. It is covered with a green coat, or skin, 
called the cuticle. Leaves are furnished with pores called 
stomMa^ for exhaling and inhaling gases. They present to the 
air a more extended surface than all the other vegetable organs, 
and are of great importance to the vitality of the plant, by im- 
bibing suitable nourishment, and throwing off such gases as 
would be useless or injurious. In other words, leaves are 
organs of digestion and respiration. 
a. We have seen how the bud is formed, and by what wise means the principle 
of life which it contains is protected through the cold and dampness of winter. In 
the spring, when the sun having reci-ossed the equator is advancing toward our 
hemis[Jiere, the vegetable world quickened by its influence begins to awaken 
from a dormant state, the buds expand, and bursting their envelopes, the new 
branches bearmg leaves and flowers come forth. 
51. The arrangement of 
the leaf in the bud is call- 
ed vernation {mr., spring), 
prefoliation {prce^ be- ; 
fore), and gemmation ri 
{gemma ^ a bud) ; it dif- 
fers in difi*erent species, 
but in each follows a 
regular law. 
Figure 37, at a, shows a young 
leaf of the currant ; this is fold- 
ed. At 6, is a young leaf of the 
monk's, bood ; this is injlectcd. At c, is the young leaf of a fern { Aspidium)', this is 
circhtatt^ or rolled from the summit toward the base. 
Fig. 37, 
Origin of leaves — Complete leaf — Different strata of the leaf. — 50. Structure of the leaf— Utiliti 
4f leaves to the plait — a. jExpanding of the bail,- -51. Arrangement of the leaf in tlie hud, Fig. 37. 
