354 MR. L. LEWTON-BRAIN ON THE ANATOMY 
Cynosurus echinatus is a vare grass in Britain, and I have not been able to obtain it 
from its natural habitat. The leaf-structure is practically that of a typical wood-grass 
(compare Pl. 39. fig. 51 with Pl. 38. fig. 32 &c.). We get a thin broad leaf, obsolete 
ribs on the upperside, feeble cutinization of the epidermal cells, stomata present on both 
sides, and broad bands of motor cells. Iam unable to give any satisfactory explanation 
that will account for such a leaf-structure in a maritime sand-grass. 
Practically the same may be said of Bromus maximus, another rare grass in Britain, 
It may, however, be recalled that the leaf-structure is very constant throughout the 
genus Bromus. 
Hordeum maritimum and Phleum arenarium are both small plants, with a short 
vegetative period and very short leaves. It need not surprise us therefore to find that 
neither shows any xerophilous characters in the structure of the leaf. 
v. Grasses of Moors and Heaths. 
Grass. Duration. Figure. 
NEE nk ga x» mk Perennial, 3, 4 
Nardus stricta » 2 
Agrostis canina . » 19 
edel e T lo ee ae VS Së 34 
ASIN COMI OR DI iL TR » 29 
Aira flexuosa . 6 
22 
The typical heath-grasses live under conditions which are most decidedly xerophytie, 
are generally found in places where they have no shelter, and are constantly exposed to 
high dry winds and to the sun, where consequently the conditions for rapid transpiration 
are most favourable, and where also the water-supply is limited. 
In agreement with this, we find that the typical leaf-structure is perhaps the most 
pronouncedly xerophytic met with in British grasses ; certainly it differs more markedly 
from the meadow-grass type more than does any other. 
Three out of the six grasses of this group, which I have examined, possess this 
“typical” leaf-structure. These are Festuca ovina (Pl. 36. figs. 3, 4), Nardus stricta 
(fig. 2), and Aira flexuosa (fig. 6); and of these Aira flexuosa possesses the most highly 
modified leaf-structure. To these, so far as structure goes, might be added the leaf of 
Lepturus filiformis (fig. 8). 
The leaf is subulate, the upper surface being reduced to a mere channel in what is 
practically a solid leaf. I have already attempted to trace the evolution of this type of 
leaf from the more usual type with a large upper surface. By assuming this form the 
leaves offer as small a surface for the cubic contents as is possible. 
The cells of the lower epidermis are very strongly cutinized and have their outer walls 
flat. Stomata are entirely absent from this lower epidermis. These two features, of 
course, render the amount of transpiration that takes place from the lower epidermis 
quite a negligible quantity. 
The fold which represents the upper surface of the leaf is covered with numerous 
short hairs; these and the position of the stomata on the sides of the fold shelter the 
