THE MATERIAL OUTGO OF PLANTS 



345 



Fig. 641. — Enlarged details of spiral {$), pitted (p), and reticulate (r) tracheae; at 

 i, 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 

 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: p, primary 

 phloem ; x^ primary xylem ; 

 c, iirst divisions of cambium 

 cells. — After Bonnier. 

 Diagrammatic. 



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- 

 posed of x^ and ^21 the 

 primary and secondary 

 xylem. — After Bonnier. 



