INTERNAL STRUCrUBE 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, hj 

 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 jjood spe- , ^'S- ^--Transverse secHon of the leaf of SUphivm 



^ lacimatum. e, epidermis of the upper surface ;«', epi- 



«imen3 of very young dermis of the lower surface ; p, palisade tissue of the 



leaves Bv rarpfnllv J^PP"* portion of the leaf; p', palisade tissue of the 



ieaves. ay caremuy j^^g^ part of the leaf ; s, a stoma seeu iu transverse 



removing the outer section. X 235. —From a drawing by the author, 

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



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

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

 1877, p. 480. 



