THE MATERIAL OUTGO OF PLANTS 
345 
FIG. 641. Enlarged details of spiral (s), pitted (p), 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 
P 
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, 
forming by division both 
secondary phloem and 
xylem; #, xylem, com- 
posed of % and x t , the 
primary and secondary 
xylem. After BONNIER. 
