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. 
(b) 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- 
spicuous concave upper surface. 
©  Motor-cells between each pair of ribs: sclerenchyma 
not forming a continuous layer below. 
= Ridges at least 5—6 times as high as the leaf- 
thickness between. 
Aira cespitosa. 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). 
