THE MATERIAL OUTGO OF PLANTS 



345 



FIG. 641. Enlarged details of spiral (s), pitted (/>), and reticulate (r) 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 



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, first divisions of cambium 

 cells. After BONNIER. 

 Diagrammatic. 



FlG. 643. Older vas- 

 cular bundle, with second- 

 ary thickening in progress. 

 p, phloem ; c, cambium, 



xylem; x, xylem, com- 

 posed of Xi and x 2 , the 

 primary and secondary 

 x. After BONNIER. 



