STRUCTURE OF SUBMERSED LEAVES. 146 
immediately beneath the epidermis of the upper surface of the 
leaf is sometimes specially modified, as in the leaves of the 
Coniferze, where its cells become elongated and sclerenchyma- 
tous, when it constitutes a form of the hypoderma (see pages 48 
and 143). The parenchyma varies in amount in different leaves; 
thus, in ordinary leaves it is moderately developed, and the 
leaves are then thin and flattened ; while in other leaves it is 
formed in large quantities, when they become thick and fleshy, 
and are termed succulent. In ordinary flat leaves all the cells 
composing the parenchyma are commonly green from containing 
chlorophyll granules ; but in succulent leaves the cells in the 
centre of the parenchyma are usually colourless. 
The parenchyma also varies in the form and arrangement of 
its component cells in different parts of the same leaf: thus in 
ordinary flat leaves we find beneath the epidermis of the upper 
surface one (fig. 278), two, or three layers of closely packed 
oblong or somewhat elongated cells (jig. 277, ps), and forming 
the tissue which has been termed palisade parenchyma. The 
form and arrangement of the cells beneath the epidermis of the 
lower surface are commonly entirely different ; thus, here the 
cells (fig. 278) are loosely connected and have numerous large 
spaces between them ; they are also frequently very irregular 
in form, presenting commonly two or more projecting rays, 
which become united with similar projections of the cells next 
them, and thus leave numerous interspaces which communi- 
cate freely with each other, and form a spongiform parenchyma 
(fig. 124, c). These interspaces are also connected with the 
stomata, which, as we have already seen, are generally most 
abundant on the epidermis of the lower surface, and thus a 
free communication is kept up between the interior of the leaf 
and the external air, which is essential to the due performance 
of its functions. 
Such is the general arrangement of the parenchyma in the 
blades of aerial leaves, but it is subject to various modifications 
in those of different plants. Thus in blades which have their 
margins turned upwards and downwards instead of their sur- 
faces, the arrangement of the parenchyma is similar beneath 
the epidermis of both the surfaces ; while in succulent blades 
the parenchyma is composed of cells which are usually larger 
than those in the blades of ordinary leaves, and closely com- 
pacted, or with but few interspaces. In the floating leaves of 
aquatic plants, again, the spongiform parenchyma is beneath 
the epidermis of the upper surface of the blades, and the com- 
pactly arranged cells next that of the under surface, the position 
of the parts being here therefore completely reversed. 
2. SuBMERSED LEAvEs.—The petiole when present in these 
leaves is solely formed of parenchymatous cells, which are, 
however, frequently elongated ; and the blades are therefore 
also necessarily entirely formed of parenchyma, the so-called 
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