356 MR. L. LEWTON-BRAIN ON THE ANATOMY 
To these grasses, Aira cespitosa from wet meadows might perhaps be added. Its 
leaf-structure is exceptional for any group of grasses, but it shows some resemblances 
in this to Alopecurus geniculatus. 
The conditions under which these grasses grow may easily be described: the water- 
supply is abundant, and they are sheltered on the banks of streams, &c., from rapid 
transpiration. As in most water-plants, the conditions are not favourable for aeration. 
The two species of Glyceria (Pl. 38. figs. 37, 39) show perhaps the most typical 
leaf-structure. The modifications from the meadow type all tend to secure more rapid 
transpiration and more efficient aeration. We get, then, a large leaf, broad and long, 
with both surfaces flat or nearly so, while the stomata are abundant and about equally 
numerous on the two sides: these characters all favour transpiration. 
Owing to their sheltered situations the stereome is not abundant; the numerous small 
supernumerary bands of stereome in the leaf of G. aquatica serve to support those parts 
of the leaf which contain the large air-lacunze. The lacunze themselves serve, of course, 
to facilitate aeration. 
Of the others, Catabrosa aquatica is quite typical in leaf-structure except for the 
absence of large air-spaces; this, however, is partly compensated for by the very loose 
and spongy arrangement of the mesophyll-cells. Like that of Glyceria fluitans (Pl. 38. 
fig. 39), the cells of the upper epidermis are papillose. 
The reed-grasses Digraphis arundinacea and Arundo Phragmites differ chiefly in the 
large amount and strength of the stereome present in the leaf. They are tall grasses 
and their broad leaves are therefore much exposed to the wind; that is, to p 
strain, as may easily be seen by watching a clump of either of these grasses in a light 
wind. The absence of lacunæ is probably dependent upon the same fact; by the greater 
exposure of the leaves more efficient aeration is obtained ; moreover, the presence of 
large lacunze would perhaps render the leaf too liable to be torn. 
Alopecurus geniculatus differs mainly in the presence of high well-marked ribs on the 
upperside of the leaf. These occur even more strikingly in Aira cespitosa, and it is 
very difficult to understand their significance. It may be noted that the grooves 
between the ribs are much wider here than they are in the leaves of the maritime sand- 
grasses. Large air-laeunz, again, are not present in Alopecurus geniculatus, but in all 
other respects the leaf-section agrees with types. 
vii. Alpine Grasses. 
Grass. Duration. Figure. 
Phleum alpinum . i i Perennial. 52 
Alopecurus alpinie =r 72-0 ae RG P 25 
Pos GUNG ou. V $3 uou E 33 
Léi 
, var. vivipara . 
Poa nemoralis, var. Balfourii 
Avena alpina . 
23 
3) 
2 
The leaf-structure of the grasses of this group possesses the distinction of being the 
less adapted to its environment than is that of any other of our cecological groups 
