194 
AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
i 
distinguished. In the first of these the cymose structure is obvious, each 
cyme is more or less elongate with secund heads, and branches occur at 
such intervals that the scorpioid structure is not obscured. In the second, 
secondary branches are developed at the bases of a great many heads, so 
that three successive nodes without branches rarely occur. The result 
is a large branching cluster which bears little superficial resemblance to the 
Fig. i. Migration and evolution of the leafy-bracted Vernoniae of North America. 
Solid lines show distribution by their location, migration by the direction of the arrow. 
Dotted lines show probable connection by evolution between species-groups. 
simple scorpioid cyme, although undoubtedly derived from it. In the 
third, the heads are suppressed at those nodes where secondary branches 
are developed. Since these appear at virtually every node, only terminal 
heads are produced on the cymes, and the whole cluster appears to be 
dichotomously branched. The fourth stage represents a much greater 
step forward. Here the basal internodes of the inflorescence are much 
shortened or almost suppressed, while the number of heads is greatly re¬ 
duced. Since the terminal internodes retain a normal length, the whole 
inflorescence appears subumbellate. While in the first three stages new 
vegetative branches may arise from below the inflorescence, so that the 
stem may live several years and reach a large size, in the fourth type, as 
well as in the fifth, the appearance of the inflorescence prevents further growth 
