OF THE LEAVES OF BRITISH GRASSES. 319 
3. Leaves in which the upper surface is marked by very distinct and high ridges. 
Not many grasses possess leaves of this type, but as examples I may mention Psamma 
arenaria (Pl. 36. fig. 12), Agropyrum junceum (Pl. 36. fig. 13), and Aira cespitosa 
(P1. 37. fig. 20). 
Raunkiær describes the type as “ strongly xerophilous," and says that it is found zz in 
grasses on dry sunny spots, where the outer conditions for very strong transpiration are 
sometimes present, but the conditions for water-supply from the earth are very unfavour- 
able.” This is very true if we exclude Aira cespitosa, which is found in moist shaded 
meadows, that is, under conditions where transpiration is always small and where 
the water-supply is abundant, thus giving us another example of the difficulty of 
generalizing. Excluding Aira cespitosa, the grasses with leaves of this type are markedly 
xerophytie, and all are maritime grasses. The stomata are confined entirely to the 
upper surface and to the flanks of the ridges; thus even when the leaves are unrolled 
transpiration cannot be very great, and when they are rolled, as is always the case in 
hot weather, it will be reduced to a minimum. It must be pointed out, however, that 
these high ridges considerably increase the surface-area of the upperside of the leaf, 
and consequently allow of the development of a larger number of lines of stomata; thus 
Pée-Laby says that in Aira cespitosa the number of lines is increased fourfold by the 
development of these high ridges. It may be that in the development of a large number 
of stomata, the broad grooves between the ridges, and the absence of any considerable 
number of hairs which would prevent a free passage of air between the ridge, we have 
the explanation of the leaf-structure of Aira cespitosa. 
In most of the leaves of this type the stomata are confined to the upper surface—to 
the flanks of the ridges. 
4. Leaves in which the upper surface is reduced to a mere fold in the almost solid leaf. 
This type is seen at its highest development in the leaves of Aira flexuosa (PI. 36. 
fig. 6); other examples of it are Nardus stricta (Pl. 36. fig. 2) and Festuca ovina (Pl. 36. 
figs. 3, 4). Also to a certain extent we may include the leaves of some varieties of 
Festuca duriuscula (Pl. 37. fig. 17), and the lower leaves of the heterophyllous variety of 
Festuca rubra (Pl. 36. fig. 7). 
This form of leaf is highly specialized and strongly xerophytic. The grasses with 
leaves belonging here are all more or less heath-grasses, growing in dry wind-swept 
localities, where, although the water-supply may not be excessively small, the trans- 
piration is exceedingly strong. The stomata are confined to the small, well-sheltered, 
upper surface. 
Leaves of this type are probably derived phylogenetically from leaves of our second 
type. This is well shown in the various subspecies and varieties of Festuca ovina. The 
flat upper leaves of Festuca rubra and those of some specimens of F. duriuscula 
(Pl. 37. fig. 17) belong clearly to the second type. In the lower leaves of the hetero- 
phyllous F. rubra (Pl. 36. fig. 7), and in those of some specimens of F. ovina (Pl. 36. 
fig. 3), the midrib has become comparatively more important and larger, while the 
lateral ribs are fewer and, proportionately to the midrib, smaller, though still obvious, 
and the leaf is generally folded. Finally, in the typical F. ovina (Pl. 36. fig. 4) the 
