Rydberg : Rocky Mountain flora 459 



Achillea arenicola Heller, Muhlenbergia 1: 6i. 1904 

 This resembles in many respects the preceding but is much 

 more copiously villous than any of the other species. The in- 

 florescence is compact and of many large heads. The leaf-seg- 

 ments are short and crowded as in ^. lanulosa, but the plant is 

 stouter, the heads much larger and the margins darker. It 

 has been mistaken for A. horealis, but has neither the blackish 

 margins nor the acute inner bracts of that species. 



Achillea gigantea Pollard, Bull. Torrey Club 26: 370. 1899, 

 and 



Achillea laxiflora Pollard & Cockerell, Proc. Biol. Soc. 

 Wash. 15: 179. 1902, are both unknown to me. 



The species or forms discussed above are fairly distinct when 

 typical specimens from the centers of their distribution are com- 

 pared. It must be admitted that intermediate forms are not al- 

 together lacking. Intermediate forms between Achillea Millefolium 

 and A. Millefolium nigrescens have been collected in Newfoundland. 

 In northern New England A. Millefolium seems to be the only 

 species. In New York, New Jersey, and Pennsylvania, both 

 this and A. occidentalis are found, but intermediate forms seem 

 to be very rare. In the center of distribution of ^. occidentalis, 

 viz., in Wisconsin, Illinois, Iowa, and Kentucky, A. Millefolium 

 seems to be unknown, while in the north it has been collected in 

 Manitoba, Saskatchewan, Alberta, and even British Columbia. 

 It has been found also in Colorado; but judging from the localities, 

 these specimens might have been escapes from cultivation. I 

 have seen yarrows planted around miners' cabins. From Colorado 

 and Nebraska I have seen a few specimens that were somewhat 

 intermediate between Achillea occidentalis and A. lanulosa, but 

 in almost every case they could be referred either to one or the 

 other. A. lanulosa and A. subalpina both belong to the Rockies, 

 but they grow at different altitudes; the former grows also on 

 hillsides of the Great Plain region and at an altitude of 1000-3500 

 m. ; the latter only in the high mountains at an altitude of 3000- 

 4000 m. Hence at an altitude of 3000-3500 m., they are both 

 found. Here intermediate forms might be expected. In Cali- 

 fornia both Achillea lanulosa and A. calif arnica are found, but 



