96 



GENUS ARTEMISIA. 



also belongs subspecies mexicana, which is clearly a southern derivative of a ludoviciana- 

 like ancestor but marked by a very leafy habit, the foliage continuing well up into the 

 inflorescence, and by narrow leaves and lobes, these latter much elongated and with 

 finely revolute margins. It appears to be an ecologic type, but it is so extreme and as 

 far as known is so well restricted geographically that it perhaps has subspecific value. 

 According to Rydberg, the leaves of the type are not revolute, and he therefore names 

 the common form with revolute leaves as A. revoluta (see p. 86). 



In the arid portions of the southern Rocky Mountains and northern Mexico and 

 running out onto the plains to the southeast as far as Kansas is found yet another deriva- 

 tive of ludovidana, namely, subspecies wrighti. This, also, is a more xerophytic type. 

 Perhaps partly because of the more arid conditions under which it grows, and partly 

 because of the increased light intensity, the foliage has reached the extreme in the matter 

 of reduction of leaf surface. The leaves are small and divided into almost filiform seg- 



FiQ. 13. 

 Leaves of Artemisia vulgaris gnaphalodes and A. v. serrata: a, h, leaves of 

 gnaphalodes from Spring Grove, Minnesota, Rosendahl 693 (Gr); c, leaf of 

 serrata from middle portion of stem, Fountaindale, Illinois, Bebb (Gr). All 

 X0.8. 



ments. Because of this notable narrowing of the lobes, the various forms here included 

 under wrighti have been commonly treated as one or more distinct species, even by 

 conservative systematists. But intermediate forms are so plentiful in all of the larger 

 herbaria that they need not even be cited, and in the field it becomes impossible to draw 

 any line between true ludovidana and wrighti. However, the latter may stand as a 

 subspecies to include these small and extremely narrow-lobed forms of the south. Within 

 the subspecies wrighti may be detected several minor variations, as indicated on page 81. 

 In some and perhaps in all of these, as well as in typical wrighti, the basal leaves on sterile 

 shoots are floccose with a white tomentum on both surfaces, quite regardless of the 

 nature of the pubescence on the upper leaves. Although this suggests a possible connec- 

 tion with some white-leaved ancestor, all of the other evidence points to ludovidana 

 as the source of this subspecies. 



