OF THE LEAVES OF BRITISH GRASSES. 827 
probably an adaptation to the xerophytie mode of life. It would. seem to tend to 
facilitate the distribution of the water-supply brought up by the vascular bundles. 
In certain aquatic grasses, as in Glyceria fluitans (DL 38. fig. 39) and G. aquatica 
(fig. 37), and in Molinia cerulea (fig. 29), the assimilating tissue is broken up by the 
development of large air-laeunse. "These doubtless serve for the better aeration of the 
assimilating cells. ] 
Not uncommonly, when the vascular bundle is not girdered on one side but has a 
band of stereome above (or below) it, the assimilating tissue is broken by a band of thin- 
walled colourless cells stretching from the outer sheath of the bundle to the band of - 
stereome (Pl. 37. figs. 24, 28; Pl. 39. fig. 49). 
Part II.—THE STRUCTURE OF THE LEAVES OF THE GRASSES 
EXAMINED. 
I. Leaf-section rounded, angular, or curved, not much broader than deep. Vascular bundles few and 
feeble, as are also the stomata and motor cells. 
A. Sections cordate or shield-shaped, with a channel above—i. e., subulate or permanently infolded 
leaves. 
a. No stereome apart from the three vascular bundles. Motor cells and hairs obsolete. Stomata 
chiefly on upper surface. Epidermal cells feebly chitinized and arched. 
Mreora VERNA (Pl. 36. fig. 1). Anglesey. April 1902. 
Leaf very small. Midrib slightly marked. ‘Three small vascular bundles all of third 
order. All have double sheath, but inner one is not strongly thickened. 
b. Extra-fascicular stereome developed. Motor cells in the grooves. Stomata and hairs on upper 
surface only. Lower epidermal cells strongly eutinized, upper much less so. 
i. Girders to some or all of the 5-7 vascular bundles. Numerous short blunt hairs above. 
NARDUs srriora (Pl. 36. fig. 2). Scearning Fen, Norfolk. June 1901. 
Section roughly circular in outline. Upperside with well-marked ribs and grooves, 
covered with numerous, short, blunt hairs. Vascular bundles few, of first and third 
orders. All with both sheaths well marked. Stereome abundant; bundles all strongly 
girdered to lower side, some feebly to upper. Stereome-bands also above bundles. 
Melrose. June 1901. 
Exactly as above. 
ii. No girders: stereome subepidermal only. 
* Stereome in a continnous layer. 
Festuca ovina (Pl. 36. fig. 3). Swanton Morley, Norfolk. June 1901. 
Leaf subulate; ribs few. Very few short hairs on upperside, none below. Vascular 
bundles few, inner sheath not marked. Stereome-bands below most bundles, none 
above. 
