OF THE LEAVES OF BRITISH GRASSES. 333 
§§ Stomata more numerous above. 
FESTUCA PRATENSIS. Cherry Hinton, Cambs. August 1900. 
Ribs higher than in F. elatior, of various orders. Grooves on either side of midrib 
Shallow. Lower epidermal cells flatter than upper. Bundles of three orders, corre- 
sponding to ribs; all have both sheaths fairly conspicuous. Stereome abundant ; 
girders most of bundles to lower side and forms bands above, which are joined to bundle- 
sheath by bands of colourless cells. 
tt Motor cells inconspicuous. 
§ Stomata numerous on both surfaces. 
Bromus MOLLIS. Kinghorn, Fife. May 1901. 
Bundles of three orders, all with both sheaths well marked. Stereome forms girders 
to largest bundles, and bands above and below some of others. 
$$ Stomata numerous above, few below. 
AGROSTIS CANINA (PI. 37. fig. 19). Ben Ledi. June 1901. 
Both upper and lower epidermal cells fairly strongly cutinized. Stereome forms 
girders to largest bundles above and below, other bundles with bands of stereome above 
and below; bands also just inside lower epidermis opposite grooves. 
Tt Stereome relatively feeble, and not forming true girders to vascular bundles. 
1 Motor-cell groups conspicuous. 
$ Ridges 5-6 times as high as leaf-tissue between, triangular, and each with 1-3 
vascular bundles and short asperities. Stomata chiefly on flanks of ridges. 
AIRA CJESPITOSA (Pl. 37. fig. 20). Cambridge. July 1900. 
Cells of both upper and lower epidermis well eutinized and flat. Two or three vascular 
bundles in each rib, one of first order, others of second and third. All have well-marked 
double sheath. Stereome strong, in bands at tips of ribs, below largest bundles and some 
of smaller ones, under lower epidermis opposite the motor cells. Below motor cells are 
bands of thin-walled, colourless cells, which join these to stereome. 
[Aira cespitosa is so very different in leaf-structure from the other species of the genus that it is 
practically useless for purposes of comparison. Moreover, its leaf-structure, as will be seen below, is 
not that characteristic of either meadows or wet places, although its resemblance to Alopecurus genicu- 
latus (Pl. 87. fig. 21), also occurring in wet places, ought to be mentioned; however, the characters of 
the epidermis are very different in the two leaves. A. caryophyllea and A. precoz, both found in dry 
exposed situations, have the leaf-structure practically identical. The leaves of both these grasses are 
very small, and the structure might be described as a reduced “ heath” type. Comparing the structure 
of these leaves with that of the leaf of ‘A. flexuosa, a typical heath-grass with long awl-shaped leaves, we 
see that all the distinguishing characters of the latter are in the direction of greater xerophilism. The 
upper surface of the leaf is more reduced and is covered with hairs ; the lower epidermal cells are more 
strongly cutinized, and the stereome on the lower side of the leaf is more abundant ; this last character, 
however, is probably directly dependent upon the larger size of the leaf. It is thus not difficult to 
imagine how the leaf-structure of A. caryophyllea and A. precoz and that of A. flexuosa may have been 
derived from a common type, the two former by a reduction consequent on the reduction of the leaf itself, 
the latter by increased specialization to meet the xerophilous conditions under which it grows.] 
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