188 AMERICAN JOURNAL OF BOTANY [Vol. 10, 



as a weed in tropical America and recently reported from extreme southern 

 Florida. Its further migration through the agencies of commerce is to be 

 expected. 



3. The section Stenocephalum, with coriaceous, spine-tipped involucral 

 scales, represented by several species of tropical South America, has a single 

 little-known and recently discovered species, V. jncunda Gleason, in the 

 mountains of southern Mexico. If we assume from the negative evidence 

 at hand that the section is actually absent from Central America, we may 

 infer an early migration of the section northward, followed by extinction 

 in Central America and the isolation of the single species in Chiapas. 



4. The section Lepidaploa, with membranous involucral scales and 

 ribbed achenes, includes 120 species of North America and many more in 

 South America. 



Within this large section there is still well-marked evidence of evolu- 

 tionary development in structure, illustrated most plainly by the character 

 of the inflorescence. One type of inflorescence may be assumed a priori 

 as the most primitive, and from it by successive small changes all the other 

 types may be derived. Since the center of evolution and migration for the 

 genus is considered to be tropical South America, where this primitive 

 type is largely developed, and since the succeeding stages in the modifi- 

 cation of the inflorescence occur progressively farther to the north, structure 

 and distribution complement each other, and it may be assumed with little 

 hesitation that migration and structural evolution have proceeded simulta- 

 neously; that the tropical species, while not necessarily the oldest in time, 

 are at least the most primitive in structure, and that the outlying species 

 of the temperate part of North America are both young in age and advanced 

 in evolution. It can not now be stated whether there is a similar correlation 

 between structure and distribution among the species of South America. 



In the section Lepidaploa, the inflorescence is either a scorpioid cyme 

 or some other type of cluster obviously derived from it by certain apparent 

 modifications in the original structure. In such an inflorescence each 

 head is morphologically terminal; a lateral branch, arising at the first node 

 below the involucre, terminates in a second head and bears another lateral 

 branch which behaves in the same way. There is thus produced a more or 

 less elongate sympodial axis, morphologically indeterminate in its develop- 

 ment, and with its series of truly terminal heads apparently lateral and 

 secLind along it. Since the successive lateral branches arise from nodes, 

 which are normally marked by leaves, it may be assumed at once that the 

 leafy scorpioid cyme is the primitive inflorescence, while those species in 

 which the bracteal leaves are suppressed stand relatively higher in the scale 

 of evolution. 



Each segment of such an inflorescence then consists of a basal internode 

 with a leaf at its summit and a head beyond it. The structure of the brac- 

 teal leaves varies greatly, but in general they maintain the form and pu- 



