OUTLINES OF BOTANY. 
XVII 
nally solid and of the ordinary endogenous structure 3 soon becoming hollow 
by the surface growing faster than the centre, except at the closed nodes, 
or joints, where the woody threads are complicately entangled. 
5. Tlie Leaves. 
* Their Nature and Structure. 
64. Leaves are a contrivance for increasing the green surface of the 
plant, and exposing to the light and air the greatest practicable amount of 
the green matter of vegetation (chlorophyll, 47, which consists of innu¬ 
merable rounded globules of a waxy nature, inclosed in the cells), upon 
which the light exerts its peculiar action (17). They are portions of the 
green cellular surface, usually expanded horizontally, and covered with 
the transparent skin, or epidermis, which invests and protects the surface 
generally ( 47 ), and which serves to protect the delicate subjacent tissue 
from the immediate action of the air. 
55. In Phaenogamous Plants, however, they never consist of a cellular 
system, or parenchyma (the green pulp) alone 3 but a woody system (con¬ 
sisting of woody tissue and vessels, 8 , 9) is introduced, just as in the stem 
(with whose woody system it is connected), to give needful strength and 
support, and to facilitate the transmission of fluid. This forms the frame¬ 
work or skeleton of the leaf, which usually divides the parenchyma into 
two strata, viz. an upper and an under layer. 
56. The cells of the upper layer are generally more compactly arranged 
than those of the lower, which gives to the upper surface of the leal' its 
deeper green color. Those of the lower layer are commonly so loosely 
arranged as to leave many intervening air-cavities, or passages, communi¬ 
cating with each other more or less freely throughout the interior of the 
leaf. 
57. To these the external air obtains access through numerous perfora¬ 
tions or little slits in the skin of the lower surface, and sometimes of the 
upper also, which are called the stomata, or breathing-pores! They 
are guarded by two cells, which, when straight and parallel, close the aper¬ 
ture, and, when curving outwards, open it 3 thus controlling evaporation 
and the access of the external air. They incline to open in moist air, and 
to shut in dry. 
58. The epidermis or skin of the leaf, as in other parts of the plant, may 
be furnished with hairs, bristles, &c., which in descriptions receive differ¬ 
ent names according to their size and strength. They are prolonga¬ 
tions or productions of some of the cells of the epidermis. Glandular Hairs 
and Glands are similar in origin and structure, but usually more complex, 
and their cells elaborate an aromatic, or glutinous, or some other kind of 
liquid, or excretion. The principal terms employed for the various kinds 
of hairiness or pubescence may be sought in the Glossary. We call the sur¬ 
face smooth when destitute of hairiness and roughness, or, more specifi¬ 
cally, glabrous when the absence of hairiness alone is indicated. 
* * Tlxeir Conformation and Parts. 
59. The expanded part of the leaf forms the lamina, limb, or blade. 
In many leaves the blade is expanded immediately on leaving the stem 3 
when they are stalkless or sessile. They are clasping (or amplexicaid), 
when the base embraces or nearly encircles the stem 3 perfoliate , when the 
encircling base unites round on the opposite side so as to appear as if per¬ 
forated by the stem, as in Uvularia perfoliata (p. 497) j and sheathing, when 
the lower part enwraps the stem, as in Grasses (p. 567) and Sedges (p. 515); 
in which the distinction between the sheath and the blade usually is well 
marked. Sometimes a portion of the blade of a sessile leaf appears to run 
down along the stem underneath, when it is decurrent. 
b* 
