THE LEAVES 



53 



2. The Foliage Leaves 



The structure of the green foliage leaves of the wheat plant is more 

 complicated. Covering the surface is (i) a somewhat elaborate epidermis 

 with characteristic stomata and trichomes of various forms ; within is (2) 

 the green assimilating parenchyma, (3) the conducting vascular bundles, 

 and (4) longitudinal strands of fibrous stereome or supporting tissue. 



Fie. 46. Transverse section of the leaf-blade. *, Stereome. 



FIG. 47. Transverse section of the leaf-sheath ( * 25). o. Outer ; i. inner epidermis ; 

 /, lacuna ; t, stereome ; a, mesophyll. 



(i.) THE BLADE AND SHEATH, (a) The Epidermis. The epidermis 

 of the blade of the foliage leaves is composed of a number of diverse 

 elements arranged in parallel rows along the long axis. Some of the 

 individual rows consist entirely of elongated cells placed end to end, each 

 cell appearing in longitudinal section as a narrow rectangle 1 50-300 /* 

 long and 15-20/1 wide. In other rows short square cells 15-20/1 across 

 are intercalated here and there between the long cells : characteristic 

 lines of long cells alternating with stomata are also present, and, except 

 in T. durum, trichomes or hairs of various lengths are found scattered along 

 the rows at more or less regular intervals, the whole arrangement as seen 

 in surface view being illustrated in Fig. 48. On the upper surface of the 

 blade is a series of longitudinal ridges or ribs, the lower surface being 

 almost flat. The epidermal cells covering the ridges differ somewhat 

 in form and arrangement from those over the furrows and along the edge 

 of the leaf. 



Running along the summit of each ridge is a single row of elongated 

 thick-walled and pitted cells alternating with hairs. 



On the flanks of the ridge, right and left of this central line, are three 

 to five rows of long cells often interspersed with short ones and hairs : 



