INTERNAL STRUCTURE OF LEAVES. 



393 



FIG. 186. 



this superficial layer is well seen, when, as often happens, it ad- 

 heres so closely to the cuticle, as to be carried away with this 

 when it is torn away (Fig. 

 185, <?, c). Beneath this first 

 layer of leaf-cells, there are 

 usually several others, rather 

 less compactly arranged ; and 

 the tissue gradually becomes 

 more and more lax, its cells 

 not being in close apposition, 

 and large intercellular pas- 

 sages being left amongst 

 them, until we reach the 

 lower cuticle, which the par- 

 enchyma only touches at cer- 

 tain points, its lowest layer 

 forming a sort of network 

 (Fig. 182, d, d), with large 

 interspaces, into which the 

 stomata open. It is to this arrangement that the darker 

 shade of green almost invariably presented by the superior 

 surfaces of leaves, is principally due ; the color of the compo- 

 nent cells of the parenchyma not being deeper in one part of 

 the leaf than in another. In those plants, however, whose 

 leaves are erect instead of being horizontal, so that their two 

 surfaces are equally exposed to light, the parenchyma is arranged 

 on both sides in the same manner, and their cuticles are fur- 

 nished with an equal number of stomata. This is the case, for 

 example, with the leaves of the common Garden Iris (Fig. 187) ; 

 of which, moreover, we find a central portion (d, d) formed by 



FIG. 187. 



Vertical section of the Cuticle, and of a portion 

 of the subjacent parenchyma, of a Leaf of Iris 

 germanica, taken in a transverse direction : a, a, 

 cells of the cuticle; b. b, cells at the sides of the 

 stomata; c, c, small green cells placed within 

 these ; d, d, openings of the stomata ; e, e, lacunae 

 of the parenchyma, corresponding to the stomata; 

 /,/, cells of the parenchyma. 



Portion of a vertical longitudinal section of the leaf of Iris, extending from one of its flattened 

 sides to the other : a, a, elongated cells of the epidermis ; 6, &, stomata cut through longitudinally ; 

 c, c, green cells of the parenchyma; d,d, colorless tissue, occupying the interior of the leaf. 



thick-walled colorless tissue, very different either from ordinary 

 leaf-cells or from woody fibre. The explanation of its presence 

 is probably to be found in the peculiar conformation of the 

 leaves ; for if we pull one of them from its origin, we shall find 

 that what appears to be the flat expanded blade really exposes 

 but half its surface ; the blade being doubled together longitu- 



