THE WHITE LILY 



157 



here represents the chief mechanical tissue of the stem, 

 serving to give it the necessary stiffness. The vascular 

 bundles themselves have on the whole a similar struc- 

 ture to those of the dicotyledonous type. They are 

 collateral, the inner half being the wood and the outer 

 half the phloem (see Figs. 60 and 61). Each bundle 



sh- 



p'h 



FIG. 61. Longitudinal radial section through a vascular 

 bundle of the stem of the White Lily, sh, bundle-sheath ; 

 xy, xylem, or wood ; px, protoxylem ; ph, phloem ; , 

 sieve-tube; c, companion-cell. Magnified 220. (R. S.) 



is surrounded by a special sheath of long thick-walled 

 lignified cells (&.s. in Fig. 60, sh in Fig. 61). There is 

 often a group of thin-walled cells between the sheath 

 and the inner edge of the xylem, which itself consists 

 chiefly of vessels with a few living cells among 

 them. The vessels are of the same kinds as those of 



