TTT TE TT M 
OF THE LEAVES OF BRITISH GRASSES. 337 
** Stereome restricted to isolated bands and not forming girders. Very hairy on both sides. 
BROMUS sTERILIS (Pl. 37. fig. 27). Linlithgow. June 1901. 
Cells of neither epidermis strongly cutinized. Motor cells sometimes none at sides of 
central bundle. Stereome forms half-girders below most of the bundles of first and 
second order, also above most of those of first order; smallest bundles have no bands 
above or below. 
ii. Hairs few, inconspicuous, or none. Stomata on both surfaces, but usually more numerous 
above. 
* Short sharp hairs or asperities, usually on upper surface only. 
t Stereome relatively abundant as girders to some or all vascular bundles. 
1 Motor cells restricted to two series flanking the midrib, and conspicuous. Epidermal 
cells fairly strongly cutinized, especially below. 
AVENA PRATENSIS (Pl. 37. fig. 28). Ben Lawers. July 1901. 
Midrib projects below and slightly above, grooves on either side above; otherwise 
leaf-surfaces parallel and flat. Leaf thick in comparison with breadth. Lower epidermal 
cells flat; upper arched. Stomata rather more frequent above. Bundles all have both 
sheaths well marked. Stereome forms girders below bundles nearest midrib, and bands 
above and below or only above other bundles. 
Harston, Cambs. May 1902. 
Leaf very much as above, but rather larger. There are more bundles, some of which 
have no stereome above or below. Bundle in midrib and in some others have girders 
below. 
AVENA PRATENSIS, var. ALPINA. Royal Botanic Garden, Edinburgh. May 1901. 
Leaf very broad and thick. Midrib sunk above. Stereome forms girders to some 
of the bundles, bands above and below others. 
[Except that 4. alpina possesses a much larger leaf, there is practically no difference in leaf-structure 
between it and the true 4. pratensis. The change of environment, such as it is, has had no effect on 
the structure of the leaf.] 
Poa cowPRESSA. Kinghorn, Fife. May 1901. . 
Lower epidermal cells flat, upper arched. A few short stiff hairs above, mostly over 
bundles. Bundles very numerous and close, of three orders. All have conspicuous 
double sheath. Stereome forms girders below and bands above larger bundles (but only 
bands in the midrib) and bands above or below or none to smaller bundles. 
Poa wEMORALISs. Bridge of Allan. May 1901. 
A few short hairs above over the bundles. Bundles in midrib not girdered below, 
other large bundles girdered above and below; stereome-bands above and below or only 
above smaller bundles or none. 
tt Motor cells in all intervals between ribs and conspicuous. 
§ Large air-cavities in leaf-tissue on each side of the midrib, 
MoLrINIA CJERULEA (PI. 38. fig. 20). N. Wales. July 1900. 
Leaf rather thin. Ribs square. Epidermal cells small and strongly cutinized on 
