90 YOUNG AND OLD LEAVES. 



the equitant leaves of Iris, the leaves of the Com- 

 pass Plant, 1 and the folded leaves of Grasses in 

 dry weather. Either the whole outspread or 

 rolled surface of the leaf is erect, as in most 

 bulbous plants, or the midrib of the leaf may be 

 bent towards the horizon. In the latter case, the 

 two margins make a single edge, turned away 

 from the rays of the midday sun, as in some Grasses 

 (Glyceria, Poo) and in the Cherry-tree. Some- 

 times the petiole is upright, and the tender tissue 

 is drawn down over it like a shut umbrella, as in 

 Podophyllum, and several of the Crowfoot family. 

 In the Horsechestnut the folded segments of the 

 leaves issuing from the bud are upright ; then they 

 droop, so that their points are directed to the 

 earth ; and later, when their epidermis is more 

 thickened, they raise themselves again till they 

 are nearly parallel with the ground. Sometimes 

 the upward-growing petiole is bent over in the 

 form of a bow, and the folded leaves hang verti- 

 cally on the curved end, as in the common Oxalis 

 (Oxalis acetosella) and many other plants (Fig. 

 17 8 ). 



1 The blades of the Compass Plant (Silphium laciniatum) take a 

 vertical position, by the leaves making a half twist. On the prairies 

 the direction of the leaves is usually north and south. 



