54 



THE WHEAT PLANT 



parallel to these, at the base of the ridge, are single or double lines of 

 stomata. 



In the furrow between two ridges is a band of three to seven rows of 

 elongated cells, whose walls are thinner and not so distinctly parallel to 

 each other as those of the long cells in other parts of the blade : these are 

 termed " bulliform cells " by Duval-Jouve and " motor cells " by Pe'e- 

 Laby. They are somewhat shorter than the ordinary long cells of the 

 epidermis, measuring 100-250 /z in length and 17-20 ^ in width : in 

 transverse section they are seen to be deeper and of different form from 

 the neighbouring cells, being spread out in the form of a fan, each cell 

 being narrow at the outer part, and broader or smaller at the base within 

 the leaf, where it is in contact with the assimilating parenchyma (a, Fig. 56). 



FIG. 48. Upper surface of the leaf-blade, a, Hair ; b, c, rows of stomata ; 

 e, rows of motor cells ; n, optical section of mesophyll cells ( x 105). 



When transpiration is excessive these cells lose water and the edges 

 of the lamina curve inwards, tending to check any further loss through 

 the stomata, which are abundant on the upper surface. On reabsorption 

 of water the blade becomes flat again. 



The rest of the epidermis is of fairly uniform thickness, 25-30 fj, deep, 

 each cell about as long as broad, with a convex cutinised outer wall 4-5 //, 

 thick, the inner and lateral walls being much thinner. 



Several rows of epidermal cells bordering the edges of the leaf-blade 

 have specially thick sinuous walls. 



The trichomes or hairs on the leaf-blade are always unicellular, and 

 vary much in length and stoutness ; some of them on the edges of older 

 leaves are little more than blunt prominences, others are short and stout, 

 20-30 [L long, with fine curved points rendering the surface scabrid ; on 

 soft velvety leaves of T. dicoccum and T. turgidum, the hairs often attain 



