334 MR. L. LEWTON-BRAIN ON THE ANATOMY 
§§ Ridges not more than 2-3 times as high as tissues between, and only one vascular 
bundle in each. 
|| Short stiff hairs on crests of ridges both sides. Stomata about equally abundant 
both sides. 
AGROSTIS SPICA-VENTI (Pl. 37. fig. 22). Royal Botanic Garden, Edinburgh. July 1901. 
Ribs shown on both sides; midrib the least prominent, others uniform. Epidermal 
cells arched on both sides. Stereome forms bands above and below bundles. Hairs 
fairly frequent on both sides at summits of ridges. 
[Agrostis is a genus of grasses in which the leaf-structure is very uniform. It is true that most of the 
species are characteristic of either pastures or wet places, between which groups, as we shall see later, 
there is very little difference in leaf-structure. But even A. canina, which is a heath-grass, is very little 
different from 4. vulgaris, and does not show the least approach to the “ typical” leaf-section of heath- 
grasses. A. Spica-venti has the most distinctive leaf-section, but even here the modifications do not show 
any relation to the environment. ] 
II No hairs or asperities below, but a few on crests of ridges. 
“| Chlorophyll-tissue in rings or partial rings around the vascular bundles. 
Poa RIGIDA (Pl. 37. fig. 23). Saxlingham, Norfolk. August 1900, 
Rounded ribs; lower side flat. Stomata more abundant above. A few short stiff 
hairs above, none below. Bundles have double sheath, but inner is not well marked, 
except round central bundle. Stereome in small bands above and below some of bundles, 
above others. Chlorophyll-tissue in rings around bundles. 
Poa LOLIACEA. Newquay, Cornwall. July 1902. 
Upperside with well-marked, rounded ribs (higher than in P. rigida). Lower side 
flat. Cells of both upper and lower epidermis strongly cutinized (those of lower rather 
the stronger) and flat. Stomata on the flanks of the ridges. Bundles of two orders, all 
with well-marked double sheath. Stereome-bundle in midrib feebly girdered below, others 
have bands above only. Chlorophyll-tissue more or less in rings about bundles (not so 
strongly marked as in P. rigida). 
[As regards leaf-structure we must divide the species of Poa into two groups: the first includes Poa 
annua, pratensis, compressa, trivialis, nemoralis, bulbosa, and alpina ; the second includes Poa rigida and 
loliacea. The last two species, moreover, are often removed from this genus and placed either in Festuca 
or Sclerochloa, and their leaf-structure certainly resembles that of some species of Festuca. Taking only 
the first group into consideration, we have an excellent example of how the leaf-structure may remain 
constant through several allied species, in spite of great differences in the environment. We have 
representatives here of meadow, waste-place, alpine, maritime, and wood grasses, and yet it is practically 
impossible to recognize any one from its leaf-structure, the only species that it might be possible to recognize 
being Poa bulbosa and P. alpina. The former differs chiefly in the smaller size of the leaf, the latter in the 
greater thickness of the leaf and the stronger cutinization of the epidermal cells. P. alpina, then, may 
be said to have its leaf-structure modified to fit it for its environment; but this is the only case in the 
group; and even here the modification is very slight. The differences between the leaf-sections of the 
other species are of minor importance and are all liable to variation. 
Poa rigida and P. loliacea are both xerophilous in habitat, and the leaf-structure is very similar, 
