336 MR. L. LEWTON-BRAIN ON THE ANATOMY 
[We have in the genus Alopecurus representatives of four natural groups of grasses—meadow, way- 
side, alpine, and water. The leaf-structure in each shows a strong family resemblance to the others, the 
most striking difference being the higher ribs of A. alpinus and A. geniculatus; in neither case are high 
ribs characteristic of the habitat. Comparing the others with A. pratensis (meadow) we see that 
A. agrestis (wayside) shows practically no difference, except that it possesses rather less stereome ; 
A, alpinus differs in the height of the ridges and also in the much smaller amount of stereome present, 
the latter feature may be regarded as an adaptation to its environment. A. geniculatus, again, differs in 
the higher ribs and much smaller amount of stereome, but also in the greater abundance of stomata on 
both sides, and in the loose arrangement of the chlorophyll-tissue; the last three features certainly can 
be looked upon as adaptations to moister conditions of life. Probably the resemblance between A. alpinus 
and 4. geniculatus is connected with the similarity of habitat, though the former is found only in alpine 
wet places, while the latter is a lowland form.] 
B. Ridges inconspicuous or obsolete. 
a. Midrib evident or prominent as a keel below. 
i. Hairs conspicuous in number or length or both.  Stomata on both suríaces, but more 
numerous above. 
* Stereome relatively prominent as girders to all or most vascular bundles. 
t Motor cells between all the ribs, except the midrib and those flanking it. Epidermal 
cells of lower surface strongly cutinized, upper less so. 
BROMUS ASPER (Pl. 37. fig. 26). Edinburgh. July 1901. 
Midrib projects below, otherwise leaf-surfaces practically parallel. Strong hairs; 
some short, others long, generally above or below a bundle. Three bundles in midrib. 
Bundles of three orders, sheaths not always conspicuous. Stereome forms girders to 
all the larger bundles, at least below. 
Cambs. Aug. 1900. 
Similar to above. 
[See also B. sterilis, p. 337.) 
tt Motor cells between all the ribs. Hairy above, and stomata much more numerous 
there. 
i Motor cells conspicuous. 
BRACHYPODIUM SYLVATICUM. Cambs. June 1900. 
Leaf very thin in comparison with breadth. Lower epidermal cells very strongly 
eutinized and with flat outer surfaces. Upper cells much less so and arched. Fairly 
abundant, short, sharp hairs and a few long stout hairs, mostly on upperside. Bundle- 
sheaths not well seen in smaller, but fairly conspicuous in larger ones. Stereome 
girders most bundles below and bands above bundles. : 
ii Motor cells inconspicuous. 
Merica nutans. Linlithgow. June 1901. 
Leaf very thin and broad. Hairs abundant above. Cells of neither epidermis strongly 
cutinized, those of lower rather more so than upper; all the cells are somewhat arched. 
Stereome forms girders to all bundles above and below. 
