71 
PHYSIOLOGY OF THE LEAF. 
The- covering of this frame-work of the leaf is first, a pulpy 
substance, called the parenchyma or cellular tissue. Some 
leaves contain much more of this than others, of course they are 
more pulpy and juicy; it is found, as its name cellular would 
denote, to consist of a mass of little cells, various in size in dif- 
ferent leaves ; in some, with the most powerful magnifiers, the 
cells are scarcely perceptible ; in others, they may be seen with 
the naked eye. These cells are of important use, in the secre- 
tion and communication of substances through the leaf; and 
may thus be considered as a kind of glands, having a communi- 
cation with the vascular system. 
The covering of the leaf, is called the cuticle or epidermis ; 
this guards the vascular system and cellular tissue from injury, 
and is the medium, by which the leaf performs the important 
functions of absorbing nourishment, and throwing off such sub- 
stances as are useless or hurtful. The cuticle is sometimes cov- 
ered with downy or hairy glands, which seem to afford securi- 
ty against changes of weather ; such plants are capable of en- 
during a greater degree ofheat than others. In some cases the 
cuticle is covered with a transparent varnish, that is very important, 
preserving the plant from injury by too much moisture, adding 
also to its beauty. The trees of Abyssinia and some other coun- 
tries, which are subject to long rains, and continued moisture, 
are thus shielded from the injurious effects of the weather. 
Some of the uses of Leaves. 
Leaves perforin a very important office, in sheltering and pro- 
tecting the flowers and fruit ; the fact of their inhaling or ab- 
sorbing air, is thought to have been proved, by placing a plant 
under an exhausted receiver, permitting the leaves only to re- 
ceive the influence of air ; the plant remained thrifty in this situ- 
ation for a length of time ; but as soon as the whole plant was 
placed under the receiver, it withered and died.* 
The upper surface of leaves, usually of a deeper green, is sup- 
posed to perform a more important part in respiration, than the 
under surface. The upper surface is also thought to repel mois- 
ture ; as you may perceive upon a cabbage leaf after a shower, 
or heavy dew, that the moisture is collected in drops, but has 
no appearanee of being absorbed by the leaf. It has been found 
that the leaves of plants, laid with their upper surface upon the 
water, wither almost as soon, as if exposed to the air ; although 
*1 give this experiment on the authority of Barton ; but although the re- 
spiration of leaves seems not to be doubted, this experiment may not be 
thought a fair one ; for it would seem very difficult, to place a plant under 
a receiver, with the leaves exposed to the air, without at the same time ad- 
mitting any air into the receiver. 
Cellular tissue— Cuticle— Leaves inhale air— Upper surface.— 
