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 

 phloem until finally it surrounds it 

 completely. All the intermediate 

 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. 549. — Transverse section of stem of 

 dicotyledon (box elder), showing vascular 

 cylinder composed of three growth (annual) 

 rings of xylem formed by the cambium. 



Fig. 550. 



-Transverse section of stem of monocotyledon (corn), showing the 

 " scattered" vascular bundles, 



