20 SHAPES OF LEAVES [CH. 



In the following native grasses the /orm of the lamina 

 affords a useful character. 



The base tapers to the sheath below — i.e. the leaf is 

 more or less linear-lanceolate — in Molinia, Brachypodium, 

 Melica, Milium, Kosleria, and the very rare Hierochloe; 

 less distinctly so in Bromus asper and species of Hordeum. 

 The base is rounded in Arundo. In the following cases 

 the leaves are setaceous, due to the very narrow blade 

 remaining permanently folded or inrolled at its edges, and 

 usually being thickened and hardened also (Figs. 13 and 

 18). The habitat of these moor- and heath-grasses suggests 

 that these are no doubt adaptations to prevent excessive 

 evaporation by the exposure of too large a surface — e.g. 

 various species oi Aira, Festuca ovina, F. Myurus and allies, 

 Nardus, and several other species; whereas, conversely, 

 the thin flat leaves of shade-grasses facilitate exposure to 

 light and transpiration. In Avena pratensis and Agrostis 

 canina some of the leaves are involute and subulate, and 

 the thickened leaves of Poa maritima also are turned up 

 at the edges, and are U-shaped in cross-section. 



As we shall see later the degree of inrolling of many 

 grass leaves varies with circumstances. 



In most others the blades are either flat (Figs. 8 — 12), 

 or more or less conduplicate on the mid-rib. The latter 

 case occurs, for example, in grasses with flattened shoots, 

 especially at the lower part of the blade — e.g. Lolium 

 perenne, Dactylis, Glyceria, and some species of Poa, and 

 the cross-section of the leaf below, j ust before it enters the 

 sheath, is V-shaped. In Glyceria the leaf-bases may show 

 yellow or brownish triangles. 



