330 MR. L. LEWTON-BRAIN ON THE ANATOMY 
+ Stereome as a thick continuous layer below, with. strong girders and half-girders to the 
vascular bundles. 
i Horizontal girders in large ribs. 
PsAMMA ARENARIA (Pl. 36. fig. 12). N. Wales. July 1900. 
Ribs conspicuous and unequal, from 3 times to 14 times the thickness of the leaf 
between. Tops of ribs rounded. Stomata confined to flanksof ridges. Numerous hairs 
on upperside only. Bundles of three orders, corresponding to ribs. All have very 
strong inner sheath and an outer sheath. Stereome very abundant; it forms a strong 
subepidermal layer on underside; girders above and below to most bundles, also side- 
girders to some of the larger bundles. Chlorophyll-tissue restricted in amount. 
[The most “ typical” of British maritime grasses in its leaf-structure (compare with Agropyrum 
junceum and with Elymus arenarius). 'The lateral girders of stereome to the bundles are peculiar to 
this leaf.] 
tt No horizontal girders. 
AGROPYRUM JUNCEUM (Pl. 36. fig. 13). N. Wales. July 1900. 
Ridges two to three times as high as leaf-thickness between, more uniform than in 
Psamma. Larger ribs with square, others with rounded tops. Cells of lower epidermis 
flat. Numerous rather short hairs above only. Bundles large, of three orders, corre- 
sponding to ribs; all have very well-marked inner and outer sheaths. Stereome 
abundant as a subepidermal layer on underside, and half-girders to larger bundles below ; 
also bands above bundles connected in large ribs to bundle-sheath by bands of colourless 
cells. 
[Although A. junceum is sometimes regarded as merely a variety of A. repens, the difference in leaf- 
structure between the two is very considerable; moreover, the differences can all be regarded as 
adaptations to the environment. The high ribs covered with hairs, the strong cuticle of the lower 
epidermis, and the absence of stomata from the lower side of the leaf of A. junceum all tend, with the 
frequent rolling up of the leaf, to the reduction of transpiration. The greater amount of stereome in 
this leaf is due probably to the more exposed conditions in which A. junceum is found, and the greater 
storms &c. to which it is subjected; the development of the subepidermal stereome-band is probably an 
adaptation to secure mechanical support when the leaf is unrolled ] 
tt Girders, but no continuous layer of stereome. 
ELYMUS ARENARIUS (Pl. 36. fig. 14). Hunstanton, Norfolk. J uly 1900. 
Upperside with well-marked ribs of two orders, of which the largest are twice the 
height of the leaf-thiekness between. Large ribs with square, others with rounded tops. 
Lower side flat. Numerous short stout hairs above; none below. Bundles large, of 
three orders, one to each rib corresponding in order to ribs. Both sheaths conspicuous 
in all bundles. Stereome abundant; all bundles girdered below and the larger ones 
above also; stereome-bands above smaller bundles. 
At e tip (fig. 9) the ribs are low and the stereome is very abundant. (See above, 
p. 329. 
