THE MATERIAL OUTGO OF PLANTS 



345 



t — s u ~ r 



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

 d, traces of original partition walls. — Adapted from Haberlandt and Tschirch. 



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 



dicotyledons. 



In case the xylem 



strands do not undergo 



individual secondary 

 the case in most monocoty- 



FiG. 642. — Young vas- 

 cular bundle : p, primary 

 phloem ; .v, primary xylem ; 

 c, first divisions of cambium 

 cells. — After Bonnier. 

 Diagrammatic. 



thickening (as 



ledons), there may be a cylinder of meristem 



which repeatedly produces new bundles, as in 



asparagus. But in all plants which produce P oscd of *» , and **• the 



. l • . . primary and secondary 



numerous leaves the increasing evaporation is xylem.— After Bonnier. 



Fig. 643. — Older vas- 

 cular bundle, with second- 

 ary thickening in progress: 

 p, phloem ; c, cambium, 

 forming by division both 

 secondary phloem and 

 xylem; x, xylem, com- 



