OF THE LEAVES OF BRITISH GRASSES. 355 
stomata from winds, and so reduce the transpiration, even on the restricted area where 
it is possible, to a minimum. 
The vascular bundles are few; there is generally one large central bundle in the 
enlarged midrib and one or two smaller lateral bundles. 
The stereome is, as a rule, not abundant; there is a tendency for it to be much 
stronger on the lower than on the upper side of the leaf, and for it to take the form of 
small subepidermal bands rather than of girders to the bundles; this is least well seen 
in Nardus stricta, where the bundles are girdered to the lower side of the leaf. This 
arrangement of the stereome is that which is mechanically most advantageous in a 
subaerial cylindrical organ. | 
Agrostis canina differs considerably from the above type in its leaf-structure. The 
leaf is not subulate, and the upper surface is marked with ribs and grooves; stomata 
are present on both sides, though they are more abundant on the upper; the cells of 
both upper and lower epidermis are strongly cutinized, and the arrangement of the 
stereome is that common in flat leaves. On the whole, therefore, the leaf-structure is 
quite of the type characteristic of pasture- and meadow-grasses, and the grass is charac- 
teristic rather of wet peaty places than of dry heaths, and it is therefore not surprising 
that its leaf-structure differs from that of the true heath-grasses. 
Molinia cærulea again is characteristic of wet heaths. The most peculiar feature in 
its leaf-structure—the large air-lacunze in and near the midrib—is thus accounted for 
by the need of efficient aeration. It shows, however, xerophilous adaptation in the 
small, strongly cutinized, epidermal cells, and it is also peculiar in the great strength as 
well as abundance of the mechanical tissue. The peculiar broadening out of the lower 
stereome-girder to the bundle in the midrib is probably due to the weakening of the leaf 
here by the development of the air-lacunæ. On the whole, then, this grass would be 
better described as a marsh xerophyte than as a heath-grass. 
Triodia decumbens, on the other hand, is, so far as habitat goes, a true heath-grass ; 
though it is also found in wet, barren land. In its leaf-structure it differs very widely 
from our types, but it cannot be referred to any other group. The leaf-section shows 
marked xerophytie characters in the thickness and cartilaginous texture of the leaf, in 
strong cutinization of the lower epidermal cells, in the stomata being more abundant 
on (though not confined to) the upper surface of the leaf, and perhaps in the small, 
closely-packed, ehlorophyll-containing cells. The stereome is also specially strong and 
fairly abundant. So far as leaf-structure goes, then, Triodia may be regarded as a 
heath-grass, with another, not so highly modified, type of leaf-section. 
vi. Grasses of Wet Places. 
Grass, Duration. Figure. 
Alopecurus geniculatus . Perennial. 21 
Calabrota e e o oo e E j 38 
Gleis Jadan > o o o os eeh E » 39 
Gain | 7. 2a. o conr » T 
Digraphis arundinacea. . . . . . + + + + » 45 
Armado Plyrapsmiles. oos oe s vu) E ove 8 e x 
SECOND SERIES.— BOTANY, VOL. VI. 3 E 
