64 EPIDERMIS [CH. 



are wide open in the dewy mornings in summer, close 

 up as the air gets dry and hot ; and any such leaf may 

 be seen to roll up after plucking and can be reopened 

 by moistening it. 



Fig. 22. Transverse section of left-half of leaf of Festuca elatior, var. 

 pratensis ( x about 50). The ridges a,re well marked and flattened 

 above. The vascular bundles of two orders are girdered below, but 

 only slightly above. There is no keel. There are well marked motor- 

 cells — not shown in the figure — in each groove. 



The epidermis of grasses has been closely investigated 

 by G"rob, but unfortunately his results concern very few 

 of our native species. The principal elements are ordinary 

 elongated cells, with plane or sinuous walls, various kinds 

 of short cells intercalated between the ends of these, several 

 forms of papillae, hairs, Szc. and stomata. 



The epidermis over the parenchyma of Digraphis 

 arundinacea consists of rectangular cells with plane walls. 



Series or bands of long cells only may alternate with 

 other series where short cells intervene between the long 

 ones — e.g. Nardus. 



Nardus has some of the bands devoid of stomata, 

 but abounding in short cells, whereas others (above) have 

 stomata throughout. 



In Nardus stricta, Glyceria fluitans, Sesleria, &c., there 

 are two kinds of short cells, some siliceous, others cutinized 

 only. 



Nardus has closely appressed small 2-celled hairs bent 



