INTERNAL STRUCTURE OF LEAVES. 



157 



204. Another curious leaf structure is to be seen in 

 Stipa spartea, the Porcupine Grass of the interior ; each long 

 harsh leaf is longi- 

 tudinally channel- 

 led on its upper 

 surface, which, by 

 the twisting of the 

 basal portion of 

 the leaf, becomes 

 apparently the low- 

 er, and the chlo- 

 rophyll-bearing pa- 

 renchyma is con- 

 fined to the sides of 

 the channels (Figs. 

 135 and 136). At 

 the bottom of each 

 channel the epider- 

 mal cells are pe- 

 culiarly developed 

 into a hygroscopic 

 tissue, which, by 

 contracting, closes 

 the channels and 

 rolls the leaf to- 

 gether, as always 

 takes place in dry 

 air. 



(a) Many Monocoty- 

 ledons as, for exam- 

 ple, Iris and Indian 

 corn afford good spe- 

 cimens of very young 

 leaves. By carefully 

 removing the outer 

 leaves in succession all stages of leaf-development may be obtained. 



Fig. 134. Transverse section of the leaf of SilpUvm 

 laciniatum. e, epidermis of the upper surface ; e', epi- 

 dermis of the lower surface ; p, palisade tissue of tjie 

 upper portion of the leaf; //, palisade tissue of the 

 lower part of the leaf ; , a t-toma *eeu in transverse 

 section, x 835. Prom a drawing by the author. 



south i.e., with the leaf-planes parallel to the plane of the meridian- 

 see articles in the American Naturalist: 1870, p. 495; 1871, p. 1; 

 1877, p. 480. 



