246 
MORPHOLOGY 
so that it seems evident that the amphivasal (monocotyledon) type of 
bundle is more recent than the collateral (dicotyledon) type. 
While the transformation of collateral to amphivasal bundles is pro- 
gressing, the bundles of the cylinder become more and more disso- 
ciated; some bundles enter the pith region, the definite outline of a 
hollow cylinder is broken up, and a transverse section of the stem shows 
FIG. 551. Transverse section of amphivasal bundle of Acorus (a monocotyledon), 
showing the xylem completely surrounding the phloem. 
vascular bundles scattered through the stele (fig. 550). This develop- 
ment of pith (medullary) bundles and the disorganization of the cylinder 
is the fourth stage. 
Of course there are monocotyledons which do not pass through all 
these stages, stopping at the second (when they are like dicotyledons), 
or at the third (when the cylinder is retained but the bundles are amphi- 
vasal). There are also dicotyledons in which medullary bundles de- 
velop and the cylinder is broken up (as in Castalia, Podophyllum, cer- 
tain species of Ranunculus, etc.), and even some in which amphivasal 
