THE LEAVES 



61 



(Fig. 57) : the fibres in the middle part of the thick leaf-base remain 

 unlignified for some time after those in the upper and lower sections 

 of it have changed. 



In addition to the stereome associated with the vascular bundles 

 there is a stout band of mechanical 

 tissue 80-100 /A wide, along both 

 margins of the leaf-blade just within 

 the epidermis, which greatly assists in 

 keeping flat the edges of the leaf ; it 

 is absent from the edges of the leaf- 

 sheath. 



(ii.) THE LIGULE. The ligule, 

 often 2-5 to 3 mm. long, is an emer- 

 gence of thin-walled parenchyma 

 arising at the point of union of blade 

 and sheath, its inner epidermis being 

 directly continuous with that of the 

 leaf-sheath ; it possesses no vascular 

 tissue. 



At the base it is three or four cells 

 thick. Along the thinner upper free 

 edge many of its cells are elongated, 

 forming an irregular fringe of uni- 

 cellular hairs, each 60-80 JJL long, and 

 8-10 /A broad. Both surfaces are with- 

 out stomata or hairs, and the walls of 

 the epidermis, especially at the margins 

 and back of the ligule, frequently show 

 faint transverse scalariform pucker- 

 ing. The cells are 5-10 times as long as broad, most of them colourless, 

 but some contain small chloroplasts, and others a pinkish cell-sap. 



(iii.) THE AURICLES. The auricles consist of parenchyma ; some of 

 their epidermal cells are unicellular hairs from -5 to i mm. long. 



FIG. 58. c, Transverse section of a 

 portion of the leaf-base ( x 105) ; 

 s, stereome of bundles (cf. Fig. 

 57) ; a, longitudinal section of paren- 

 chyma cells in c ; b, calcium oxalate 

 crystals from cells in c. 



