STRUCTURE OF LEAVES 



717 



dii/ me (fig. 558, d, d), with large interspaces, into which the stomates 

 open. It is to this arrangement that the darker shade of green 

 almost invariably presented by the upper surface of leaves is prin- 

 cipally due, the colour of the component cells of the parencnyme 

 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 pareii- 

 chyme is arranged on both sides in the same manner, and their 

 epiderms are furnished with an equal number of stomates. This is 

 the case, for example, with the leaves of the common garden Iris 

 (fig. 563), in which, moreover, we find a 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 conformation of the leaM - : 

 for it' 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. 



FIG. 563. Portion of vertical longitudinal section of leaf 

 of Iris, extending from one of its flattened sides to the 

 other : a, a, elongated cells of epiderm ; &, b, stomata cut 

 through longitudinally ; c, c, green cells of parenchyme ; 

 d, (1, colourless tissue, occupying interior of leaf. 



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 be equitant. Now by tracing the middle 

 layer of colourless cells, d, d, down to that lower portion of the leaf 

 where its two halves diverge from one another, we find that it there 

 becomes continuous with the epiderm, to the cells of which (fig. 563, 

 ) these bear a strong resemblance in every respect, save the greater 

 proportion of their breadth to their length. Another interesting 

 variety in leaf-structure is presented by the water-lily and other 

 plants whose leaves float on the surface ; for here the usual arrange- 

 ment is entirely reversed, the closely set layers of green leaf-cells 

 being found in contact with the lower surface, whilst all the upper 

 part of the leaf is occupied by a loose spongy parenchyme, containing 

 a very large number of air-spaces that give buoyancy to the leaf ; 

 and these spaces communicate with the external air through the 



