60 
PHYSIOLOGY OF THE LEAF. 
tubular ; in some cases, this is ascertained by the naked ey e j in oth- 
ers, it may be beautifully illustrated by immersing the fibres of the 
leaf in some coloured Uquid ; on taking them out, they are found to 
/ contain internally a portion of the liquid ; this experiment proves 
them to be transparent^ as well as tubular. 
The covering of this frame of the leaf is the cuticle, and a pulpy 
substance, called the parenchyma, or cellular texture. 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 different leaves ; in 
some, with the most powerful magnifiers, the cells are scarcely per- 
ceptible ; in others, they may be seen with the naked eye. These 
cells are of important use 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 cuticle,^ guards the vascular and 
cellular system from injury, and is the medium by which the leaf 
performs the important functions of absorbing nourishment, and 
throwing off" such substances as are useless or hurtful. The cuticle 
is sometimes covered with downy, or hairy glands, which seem to 
afford security against changes of weather ; such plants are capa- 
ble of enduring a greater degree of heat than others. In some cases, 
the cuticle is coverpd with a transparent varnish, 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 countries, 
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 vas- 
cular net- work, the leaf is rugose, as seen at Fig. 62, a ; where, for 
every swelhng of the upper surface of the leaf, there is a correspond- 
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 cellulai 
tissue, the leaf presents the appearance of lattice-work, and is saic 
to be cancellated ; the leaves of an aquatic plant of Madagascar 
(Hydrogetonfenestralis, Fig. 62, &,) are of this kind. Another exam- 
ple of this leaf is seen in the Claudea elegans, a species of marine 
Algee, found in New Holland, (Fig. 62, c;) the veins are parallel to 
the sides, and cross the nerves. 
* The cuticle is sometimes called epidermis, from ep/", around, and derma, sk:n ; 
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 ski n is below. 
How ascertained to be tubular and transparent— Cellular texture— Cuticle— Impor- 
tant oflace of the leat— What is a rugose leaf 7— What is a cancellated leaf ?— Explain 
