MORPHOLOGY 



119 



primary medullary rays, and medulla consist of fundamental tissue, and are chiefly 

 composed of colourless paren- 

 chyma. A part, however, of 

 the tissue of the pericycle 

 may become sclerenchymatous 

 (Fig. 128 A, pc) ; sclerenchy- 

 matous elements also often 

 surround individual bundles as 

 sheaths, or accompany the 

 phloem portion in the form of 

 strands (Figs. 121, 123). When- 

 ever such a sheath of scleren- 

 chyma is developed about a 

 bundle, it is interrupted on 

 both sides of the bundle, at 

 the junction of the xylern and 

 phloem portions, by paren- 

 chymatous cells, or by cells 

 which are only slightly thick- 

 ened and lignitied. The exist- 

 ence of these unthickeiied places 

 facilitates the exchange of water 

 and food material between the vascular bundles and the fundamental tissue. When 



FIG. 129. Transverse section of an adventitious root of 

 Allium Cepa. ep, Remains of the epidermis ; ex, exo- 

 dermis ; c, primary cortex ; e, endodermis ; <x, central 

 cylinder, (x 45.) 



FIG. 130. Transverse section of an adventitious root of AlHitm Cepn. r, Primary cortex ; c, endo- 

 dermis ; p, pericycle; a, annular tracheides ; sji, spiral tracheides; sc and sc* ; scalarifonn 

 vessels; v, phloem, (x 240.) 



a common starch-sheath is not present in a stem, starch-sheaths are sometimes 



