8 THE TRUE GRASSES. 



the rolling up commences alternating from right to left 

 in the successive leaves. 



Leaf Torsion. The adult blade often exhibits tor- 

 sions ; all the broader leaves of Festuca, Holcus, Calama- 

 grostis, etc., are twisted to the left, at least in the upper 

 portions, those of Secale, Triticum, etc., to the right, and of 

 many Avenece in both directions, being twisted in one direc- 

 tion above and the other below. The leaves of many wood- 

 land grasses (Milium effusum L., Braehypodium silvati- 

 cum K,. & Sch., Festuca silv. Vill., Melica altissima L., 

 Pharus latifolius L.), also of some narrow-leaved species 

 (Gynerium argenteum Nees., Ammophila arundinacea 

 Host.), are turned 180 at the base, so that the upper 

 and under sides are reversed. In this 'case the stomata 

 are only upon the original upper side. This phenome- 

 non is probably due to the influence of light (heliotrop- 

 ism) ; and this is more certainly true in regard to the 

 remarkable sleep-movements exhibited by the leaves of 

 Olyra Guyanensis (Strephium Guyan. Brongn.). In the 

 day-time the leaves of this grass stand out from the culm, 

 and the upper surfaces are turned upwards, but at night 

 they lie close to the culm and their surfaces are at an 

 angle of 90 from the position during light. This torsion 

 is effected by means of a very short petiole. 



Nervation. The nbro-vascular bundles (nerves) of 

 the blade either enter it separately and then run paral- 

 lel, or a number at first unite with colorless parenchyma 

 to form a strong midrib at their entrance to the blade, 

 later the single bundles separate one at a time and pass 

 towards the edge of the leaf. This arrangement gives an 

 especial firmness to the base of the leaf, which has to 

 bear the weight of the entire blade. Maize, Sorghum, 

 etc., are examples of this arrangement, which is espe- 

 cially frequent in tropical grasses. The nbro-vascular 

 bundles, at least the primary ones, are accompanied on 

 the lower or on both sides by bundles of sclerenchyma 

 which are frequently united into rings ; this happens 

 especially in those species with rush-like leaves, and 

 gives them their very stiff blades. The epidermis con- 

 tains very irregular cells whose walls in many cases 



