V] LEAF-STRUCTURE 77 



Arrhenatherum avenaceum. Very rare hairs above : 

 a few blunt asperities here and there. No keel. Ridges 

 low. Girders to the primary bundles, but not very strong : 

 marginal and other sclerenchyma faint, as is also the 

 cuticle. Stomata on both faces. Motor-cells fairly de- 

 veloped between the ridges. 



Briza media. No keel, and mere traces of marginal 

 sclerenchyma. Ribs practically obsolete, but well de- 

 veloped motor-cells in furrows. Principal bundles girdered. 

 Stomata on both sides. No hairs or thickened cuticle. 



Avena fatua, Molinia and Leersia also come here. 



(&) Upper and lower leaf-surfaces dissimilar, or at 

 least not parallel, owing to the conspicuous ridges 

 and grooves above. 



(1) No stomata below. 



* Leaves flat or nearly so, or at least exhibit a con- 

 spiouov^ concave upper surface. 



© Motor-cells hetioeen each pair of ribs: sclerenchyma 

 not forming a continiious layer below. 



= Ridges at least 5 — 6 times as high as the leaf- 

 thickness between. 



Aira casspitosa. Ridges high, 7- — -10 times as high as 

 the breadth of leaf between, triangular, each with 1 — 3 

 vascular bundles devoid of girders, with an upper isolated 

 band of sclerenchyma at the acute tip, and another below 

 the principal bundle. Also small bands below each group 

 of motor-cells. Small conical asperities on the ridges and 

 below. No mid-rib. Stomata on flanks of ridges only, 

 and few motor-cells between (Fig. 23). 



