MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS. 381 



and the intermediate cellular parenchyma; portions of such sections are 

 shown in Figs. 272, 274, and 275. In close apposition with the cells of 

 the upper epiderm (Fig. 274, a, a), which may or may not be perforated 

 with stomata (c, c, d, d) we find a layer of soft thin-walled cells, con- 

 taining a large quantity of chlorophyll; these generally press so closely 

 one against another, that their sides become mutually flattened, and no 

 spaces are left, save where there is a definite air-chamber into which the 

 stoma opens (Fig. 274, e); and the compactness of this superficial layer is 

 well seen, when, as often happens, it adheres so closely to the epiderm, 

 .as to be carried away with this when it is torn off (Fig. 273, c, 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 cell not being in close apposition, and large intercellular passages 

 being left amongst them, until we reach the lower epiderm, which the 

 parenchyma only touches at certain points, its lowest layer forming a set 

 of network (Fig. 270, d, d) with large interspaces, into which the sto- 

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

 .almost invariably presented by the superior surfaces of leaves is princi- 



FIG.-275. 



Portion of vertical longitudional section of leaf of Iris, extending from one of its flattened sides 

 to the other: a, a, elongated cells of Epiderm; 6, 6, stomata cut through longitudinally; c, c, 

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



pally due; the color 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 epiderms are furnished with an equal num- 

 ber of stomata. This is the case, for example, with the leaves of the 

 common garden Iris (Fig. 275); in which, moreover, we find a central 

 portion (d, d) formed by thick- walled colorless 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 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 to- 

 gether at their upper part, but at their lower they are commonly sep- 

 arated 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 horse- 

 back, that the leaves of the Iris tribe are said to be equitant. Now by 

 tracing the middle layer of colorless cells, d, d, down to that lower por- 

 tion 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. 



