146 INSERTION AND ARRANGEMENT OF LEAVES, 
veins being composed simply of more or less elongated’ paren- 
chymatous cells. The blades of such leaves are generally very 
thin, only containing two or three layers of cells, so that all the 
cells are nearly in contact with the water in which they are 
placed. The cells are disposed very regularly and have no in- 
terspaces, but all contain chlorophyll granules. In submersed 
leaves, however, which are thickened, we find large cavities 
Fic. 279 ee 
Yi 
? 
Fig. 279. Vertical section of a leaf of a Potumogeton, highly magnified. 
i i. Air cavities. 7, Parenchymatous cells containing chlorophyll 
granules. 
which are very regular in their form and arrangement (fig. 279, 
7, 1) ; these contain air, by which the specific gravity of the leaf 
is diminished, and it is thus enabled to float in the water. Sub-- 
mersed leaves have no true epidermal layer, and no stomata, 
both of which would be useless from their being always exposed 
to similar hygrometric conditions. 
3. INSERTION AND ARRANGEMENT OF LEAVES. 
1. InseRTION.—The point by which a leaf is attached to the 
stem or branch is called its insertion. Leaves are inserted on 
various parts of the stem and branches, and receive different 
names accordingly. Thus the first leaves which are developed 
are called cotyledons (fig. 18, c, c) or nursing leaves. The coty- 
ledons are usually very different in their appearance from the 
ordinary leaves which succeed them. The first leaves which 
appear after the cotyledons are termed primordial (fig. 18, d, d); 
these, and the cotyledons, generally perish as soon as, or shortly 
after, the development of the other ordinary leaves. Leaves 
are called radical when they arise at, or below, the surface of 
the ground, and thus apparently from the root, but really from 
a shortened stem, or crown of the root as it is commonly called. 
Leaves are thus situated in what are termed acaulescent plants, 
such as the Dandelion and Primrose. The leaves which arise 
from the main stem are called cauline ; those from the branches 
ramal ; and the modified leaves arising from the base of, or 
upon the flower-stalks, bracts or hypsophyllary leaves (figs. 23 
and 24, b, b). 
When a leaf arises from the stem by means of a petiole it is 
said to be stalked or petiolate (fig. 274, p) ; when the blade of a 
