STRUCTURE, MORPHOLOGY, AND PHYSIOLOGY. 9 



contain much silica. The bands of upper epidermis 

 that lie between the nerves often present wedge-shaped 

 cells (" bulliform" ceils), 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 tumes- 

 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. 



