INTERNAL STEUCTUEE OF LEAVES. 



423 



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 Passages being 

 left amongst them, until we reach the lower Cuticle, which the 

 Parenchyma only touches at certain points, its lowest layer forming 

 a set of network (Fig. 223, 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 colour of the component 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 furnished with an equal number of 

 Stomata. This is the case, for example, with the Leaves of the 

 common Garden Iris (Fig. 228) ; in which, moreover, we find a 



Fig. 228. 



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 Cuticle ; b, b, Stomata cut-through 

 longitudinally ; c, c, green Cells of the Parenchyma ; d, d, colour- 

 less tissue, occupying the interior of the leaf. 



central portion (d, d) formed by thick- walled colourless tissue, very 

 different either from ordinary Leaf-cells or from Woody Fibre. The 

 explanation of its presence is to be found in the peculiar con- 

 formation 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 longitudinally, so that what may be considered its under 

 surface is entirely concealed. The two halves are adherent 

 together at their upper part, but at their lower they are commonly 

 separated by a new leaf which comes-up between them ; and it is 

 from this arrangement, which resembles the position of the legs of 

 a man on horseback, that the leaves of the Iris tribe are said to 



