322 MR. L. LEWTON-BRAIN ON THE ANATOMY 
heath and maritime types, they are usually absent from the lower side of the leaf, but 
are very abundant above. This is well seen, for example, in Agropyrum junceum 
(Pl. 36. fig. 18) among maritime, and Azra flexuosa (Pl. 36. fig. 6) among heath-grasses. 
In this case also they doubtless serve to hinder excessive transpiration by preventing 
a free cireulation of air in the groove, or grooves, the sides of which represent the 
transpiring surface in these grasses. 
In most grasses where these hairs are found, however, they are much less abundant 
than this, and can then hardly serve for the purpose of reducing transpiration. Some- 
times they occur more or less in connection with the bands of stereome, more commonly 
at the summits of the ridges, less often also below the bundles in the lower epidermis. 
As examples, I may mention Agrostis Spica-venti (Pl. 87. fig. 22) and Festuca bromoides. 
Often, however, the hairs are simply scattered irregularly over one or both surfaces of 
the leaf, as in Poa trivialis, Cynosurus cristatus, &c. 
Even more common are short stiff emergencies, or asperities, which can hardly be 
termed hairs; they are often abundant on the margins of the leaves, but are also found 
scattered on both surfaces. 
Mention must also be made of the short, blunt hairs found in the upper epidermis of 
Nardus stricta (Pl. 36. fig. 2; Pl. 40. fig. 64). They are very characteristic, and are 
similar in appearance to the papillze mentioned above in the upper epidermis of G/yceria 
fluitans. 
The hairiness of the leaves of any particular grass varies considerably, and can rarely 
be relied on as a distinctive character. 
The stomata of grass-leaves do not present any peculiarities of structure. In nearly 
every case the stoma possesses only two guard-cells, and is flush, or nearly so, with the 
surface (Pl. 40. figs. 58, 66, 73). They are arranged, as is well known, in longitudinal 
rows along the leaf, interspersed among the long epidermal cells. We usually find two 
or three rows of stomata close together, and next these several rows of long cells without 
stomata. 
The protection of the stomata in grass-leaves then is not secured by their structure, 
but by their position and arrangement. In strongly xerophytic grasses, stomata are 
entirely absent from the lower epidermis; they are confined to the upperside, which 
is the one protected by the rolling or closing up of the leaf. In the majority of grasses 
the stomata are more abundant on the upper surface than on the lower, but we get 
all stages between an equal number on both sides and an entire absence from the 
lower side. 
Vascular Bundles.—The vascular bundles enter the leaf separately and run parallel 
from base to apex of the blade. Minute cross-anastomoses between the parallel veins 
are not infrequent and are often seen in transverse section. 
Pée-Laby recognizes five orders of vascular bundles, distinguished by the characters of 
the xylem as follows :— 
The bundles of the first order are of the normal monocotyledonous type, with two 
large pitted or reticulate vessels and a large intercellular space, representing the position 
of the protoxylem. 
