SPERMATOPHYTES 
245 
ledon. In some monocotyledons 
this stage persists, and in these 
cases the adult stems resemble 
those of dicotyledons. 
In the third stage of development 
the collateral bundles gradually 
become transformed into amphi- 
vasal bundles; that is, bundles in 
which the xylem surrounds the 
phloem (fig. 551). This trans- 
formation is very evident, the 
xylem of the collateral bundle 
gradually extending about the FIG. 549. Transverse section of stem of 
phloem until finally it surrounds it dicotyledon (box elder), showing vascular 
. cylinder composed of three growth (annual) 
completely. All the intermediate rmgs O f xylem formed by the cambium, 
stages in this extension of the xylem 
about the phloem may be found. The amphivasal bundle is charac- 
teristic of the mature stems of monocotyledons. In the seedlings, the 
leaves, and floral axes, the bundles are collateral (the dicotyledon type) ; 
FIG. 550. Transverse section of stem of monocotyledon (corn), showing the 
"scattered" vascular bundles. 
