194 



AMERICAN JOURNAL OF BOTANY 



[Vol. 10, 



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 



CUBA 



Yunquensis 

 <i ? 



segregate 

 f ? 



Sagraeanae 

 « ? 



,Frutioosae 



Baharaenees 



BAHAMA ISLANDS \ 



HISPANIOLA 



Buxifoliae 

 < ? 



PORTO RICO 



VIRGIN IDS 



.albidaulla 



LESSER ANTILLES 



L > i\g i f o 1 i a e 



CENTRAL- AMERICA 



SOUTH AMERICA 



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 



