10 E. B. Babcoc\ 



lationship which have been used in the present study are com- 

 parative morphology, geographic distribution, and chromosome 

 number, size, and morphology, and with respect to certain 

 species additional evidence has been derived from genetic data, 

 especially on interspecific hybrids. The morphological features 

 of greatest usefulness in this group of plants are mentioned be- 

 low. Like most of the Compositae, these species are often diffi- 

 cult to recognize and highly variable or polymorphic. Many of 

 the species which have hitherto been published have had to be 

 revised because they were based on inadequate material or too 

 superficial study of the morphology. The species with which 

 this essay deals have all been carefully reviewed, if described by 

 others, and a number of new species which are included have 

 been so recognized only after critical investigation. 



The importance of geographic distribution to the student of 

 plant taxonomy has long been recognized. Studied in connec- 

 tion with the evidence from comparative morphology, the facts 

 of distribution may throw light on the phyletic relations of 

 species as well as of larger groups, and may indicate centers 

 of origin and distribution. Field studies on the variation and 

 ecology of the living plants in nature has also come to be widely 

 appreciated as an invaluable aid in taxonomy. But the individual 

 investigator is definitely restricted in the amount of such work 

 that is practicable, and, in attempting to classify the species in 

 any large, cosmopolitan genus, finds it impossible to apply this 

 method very generally. 



In recent years, however, the comparative study of the chromo- 

 somes, especially of their number and morphology, has come to 

 be recognized as a useful method of supplementing the evidence 

 from gross morphology and geographic distribution. At the 

 same time, the accumulation of a living collection makes it 



