52 
ANATOMY OF LEAVES. 
oxygen are combined. When leaves languish from disease, 
tliey give off oxygen sparingly, and the chlorophyl assumes 
some shade of yellow or red. 
LECTUHE X. 
ANATOMY AND PHYSIOLOGY OF LEAVES. THEIR USE IN THE VEGE- 
TABLE SYSTEM. APPENDAGES TO PLANTS. • 
60. The leaf, though simple in appearance, is complex in 
structure. It is composed of a cellulojr systew^^ which is an ex- 
pansion of the cellular integument ; and difibro-^ascular system, 
which arises from the medullary sheath. The outer covering, 
or cuticle, is a continuation of that of the stem. Leaves are com- 
pared to the lungs of animals ; they are organs for absorption^ 
exhalation^ respiration^ and digestion. "When leaves are wanting, 
as in the Prickly Pear {CaGtus\ the green surface of the stem 
appears to perform their office. Observe a dead leaf v/hich has 
for some time been exposed to the action of the atmosphere ; 
its sheleton^ or frame-worh^ consists of various fibers, minutely 
subdivided, which originate from the petiole. After boiling 
the leaves slightly, or rubbing them in water, the cuticle easily 
separates, and the pulp, or cellular texture, may then be washed 
out from between the meshes of the veined net-work ; thus, 
the most minute cords of the different 
vessels become perceptible, with their 
various divisions and subdivisions ; 
these form what is called \h.Q fihro-vas- 
Gular system. (See Fig. 61). Though ) 
in external appearance the organs 
which compose the ^vascular system of Vi^ 
plants are analogous to the bones 
which constitute the foundation of the animal system, yet they 
are rather considered as performing the office of veins and ar- 
teries. They are found to be tuhiilar. In some cases, this is 
ascertained by the naked eye ; in others, it may be beautifully 
illustrated by immersing the fibers of the leaf in some colored 
hquid : on taking them out, they are found to contain inter- 
nally a portion of the liquid. This experiment proves them to 
be transparent^ as well as tululan^. Leaves are l>ulpy or juicy 
according as they contain more or less parenchyma or cellular 
tissue. This tissue consists of a mass of little cells, various iu 
60 Structure of the leaf— Explain Fifj. 61— Cellular texture. 
