PHYSIOLOGICAL STRUCTURE. 97 
a. The tissue of the lamina, in like manner, must essentially 
correspond with the outer integuments of the bark, of which it 
is but an extension. That peculiar form of cellular tissue of 
which it is composed is called parenchyma. 
261. The parenchyma of the leaf exists in two layers, as 
might be inferred from the manner in which it is produced 
(221). In all those leaves which are ordinarily horizontal in 
position, one surface being upwards and the other downwards, 
these two layers are dissimilar in structure; but in those leaves 
where the lamina is vertical, as in the ins, they do not mate- 
rially differ. | 
a. The whole structure is, of course, clothed with the epi- 
dermis. . 
262. The internal structure of the parenchyma is more complicated than would 
be at first supposed. A powerful microscope is necessary for its examination. 
Let an exceedingly thin paring be taken from a vertical section of the lamina 
and submitted to the solar (or compound) microscope, in such a manner that the 
rays shall pass from section to section. Fig. 36 represents a magnified view of 
such a paring of the leaf of the lily, which may be regarded as characteristic of 
leaves in general. 
Pri 
FIG. 36.— 1, Magnified section of a leaf of the lily ; 2; of the epidermis with stomata. 
_263. The upper surface (a, a) is thus seen to consist of the flattened cells of the 
epidermis, arranged in a single layer. Just beneath this (b, b) is the more com- 
pact part of the parenchyma, consisting of a layer of oblong cells placed in such a 
position that their longer axis is perpendicular to the léaf’s surface. Next below 
we meet with the parenchyma of the lower surface (c, c), composed of oblong 
cells arranged longitudinally, and so loosely compacted as to leave larger empty 
spaces between. Lastly, we find again the epidermis (d, d) of the under surface 
with stomata (e, e), opening into air-chambers. 
a. Within all the vesicles of the parenchyma are seen adhering to the walls, 
the green globules (chilorophyll) which give color to the parenchyma, — dark 
