A. BIGELOVI. 



105 



In thus temporarily assigning bigelovi to Abrotanum the possibility of its representing 

 the starting point of the Seriphidia should be considered. The similarity to A. tridentata, 

 of this section, seems to be more than accidental. The tridentate character of the leaves, 

 for example, is almost identical in these two species and does not occur in any other 

 American species. The southerly distribution of bigelovi, along the borders of the 

 much more widespread tridentata, also suggests that it may be the precursor of this 

 species and hence probably of all of the American Seriphidia. If this conclusion is 

 correct, then this section, as generally accepted, had two origins, the other being from 

 Old World species. This polyphyletic origin would account for the fact that the Ameri- 

 can members of Seriphidium have little in common with the Eurasian members, and 

 the wide geographic gap between these two groups also would thus be explained. Since 

 such a hypothesis necessitates parallel development on the two continents in only one 

 character, namely, the suppression of the ray-flowers, there is no serious objection to its 

 acceptance, but, on the other hand, there is no immediate need of breaking the section 

 Seriphidium into two sections until this point can be better substantiated and characters 

 brought forward to serve for their differentiation. This is impossible at the present time, 

 because of the want of a thorough knowledge of many Old World species. 



The variation in the number of ray-flowers and of disk-flowers is indicated in table 7. 

 The tabulation also brings out the fact that there is but slight variation in any of the 

 characters enumerated. This constancy and apparent absence of plasticity is noticeable 

 also in the habit and vegetative characters, and tends to confirm the view that bigelovi 

 is an ancient form which, although perhaps once serving as a starting-point for the 

 Seriphidium stock, at least as to American species, is now in a quiescent condition. 



ECOLOGY. 



Artemisia bigelovi is a dwarf shrub, closely resembling low forms of A. tridentata 

 produced by overgrazing and burning. In the mixed prairie of northern New Mexico 

 and Arizona it often forms a dominant society similar to the low savannah produced by 

 sagebrush on the northern Great Plains. Like the latter, it is really a consocies post- 

 climax to the dominant grasses, and hence is most abundant in the subclimax stage 

 preceding them. It is itself dominant in low areas and on rocky ridges where it is 

 gradually replaced by the grasses as the succession advances. It is especially character- 

 istic of cedar savannah, owing to the generally rocky soil. It is regularly associated 

 with Hilaria jamesi, Sporobolus cryptandrus, and Bouteloua gracilis, but gradually disap- 



