20 SHAPES OF LEAVES [CH. 
In the following native grasses the form of the lamina 
affords a useful character. 
The base tapers to the sheath below— Le. the leaf is 
more or less linear-lanceolate—in Molinia, Brachypodium, 
Melica, Milium, Keleria, 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 of Avra, 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. Loliwm 
perenne, Dactylis, Glyceria, and some species of Poa, and 
the cross-section of the leaf below, just before it enters the 
sheath, is V-shaped. In Glycerza the leaf-bases may show 
_yellow or brownish triangles. 
