STRUCTURE, MORPHOLOGY, AND PHYSIOLOGY. 9 



contain much silica. The bands of upper epidermis 

 that lie between the nerves often present wedge-shaped 

 cells ("' bulliform" cells), arranged in the form of a fan, 

 whose growth and expansion causes the blade to open 

 out; in those leaves which are folded in the bud these 

 cells are only to be found on each side of the midrib. 

 In grasses that do not have such fan-shaped cell-groups 

 the blades remain always folded or rolled up, or at most 

 open but a little, and consequently appear thread- or 

 rush-like (many species of Stipa, Festuca, and Nardus). 

 In others, especially in many of the Steppe grasses, the 



FIG. 3a. Transverse section of the primary vein of Festuca elatior, var. arundi- 

 nacea. Sc. selerenchyma; P, colorless parenchyma; Cb, bulliform cells. (After 

 Hackel, Monog. Festuc. Europ., Tab. II. Fig. 6.) 



blades roll up whenever these cells lose their turges- 

 cence by excessive evaporation, but become flat again, 

 or at least half open, as soon as the air becomes moist. 

 This rolling up of the blade acts as a protection against 

 excessive evaporation ; for the lower side, which is then 

 alone exposed to the air, has especial protective arrange- 

 ments against transpiration in its strong cuticle, seleren- 

 chyma masses, etc. 



The stomata of grass leaves usually stand in longi- 

 tudinal rows and are surrounded by four cells ; two large 

 secondary cells project beyond and cover two long and 

 narrow guard-cells, which are usually much narrower 

 than the first. 



