THE LEAVES 



59 



usually with square ends and a few simple pits ; the outer or "parenchyma 

 sheath " is more conspicuous, especially round the smaller bundles, and 

 consists of thin- walled cells, often almost circular in transverse section, 



FlG. 54. Fine strand of tracheids crossing from one 

 large bundle to another (optical view of portion 

 of leaf cleared with chloral hydrate ( x 135). 



FIG. 55. Transverse section of the 

 leaf through a small vascular 

 bundle (xaio). s, Stereome ; 

 i, inner ; o, outer bundle sheath. 



measuring 20-30^ across and 100-150 /z in length, their long axes parallel 

 to the long axes of the leaf. It only partially encircles the bundle, being 



O 



FIG. 56. Transverse section of a leaf through a large vascular bundle ( x 210). a, Motor 

 cells ; 5, stoma ; n, mesophyll cells ; b, phloem of vascular bundle ; o, outer sheath 

 of bundle. 



discontinuous on the phloem side, and in the larger bundles is united to 

 the girder of stereome mentioned below. Unlike the ordinary parenchyma 

 of the blade, the outer sheath contains few or no chloroplasts except when 



