THE MATERIAL ()UT(!() OF PLANTS 345 



f — s •" - ■ r 



Fig. 641. — Enlarged details of spiral (s), pitted (/>), and reticulate (r) tracheae; at 

 d, traces of original partition walls. — Adapted from IIabeklanut and TSCHERCH. 



ones. When numerous primary and secondary 

 strands are produced, they may form a column 



of xylem, with pith in 



the center, interrupted 



by thin radiating plates 



of parenchyma, the 



pith rays. Such is the 



condition in the sun- 

 flower, castor bean (fig. 



644), and many other 



di( otyledons. 



In case the xylem 



strands do not undergo 



individual secondary 

 thickening (as is the case in most monocoty- 

 ledons), there may be a cylinder of meristem 

 which repeatedly produces new bundles, as in 

 asparagus. But in all plants which produce 

 numerous leaves the increasing evaporation is 



Fig. 642. — Young vas- 

 cular bundle: />, primary 

 phloem ; .v, primary xylem ; 

 r, first divisions of cambium 

 cells. — After Bonnikk. 

 Diagrammatic. 



Fig. 643.— Older vas- 

 cular bundle, with second- 

 ary thickening in , 

 p, phloem ; e, cambium, 

 forming by division both 

 secondary phloem and 

 xylem; .v, xylem, com- 

 post '1 of .Vj and .v., the 

 primary and secondary 

 xybm. — Ai't.-r Bonnier. 



