144 GENUS ARTEMISIA. 



from those growing on the coastal slopes, or this may be only the result of unfavorable 

 conditions obtaining in that bleak, wind-swept, and more elevated valley. The most 

 congested inflorescences are on plants collected in 1919, after three years of exceptionally 

 low rainfall {Hall 10959, UC). In the same year the plants on the coastal slopes had 

 well-developed inflorescences. A parallel reduction in the inflorescences of typica is 

 also common (for example. Granger, Wyoming, September 12, 1919, Hall, UC). From 

 these various considerations it is seen that parishi bears all the evidence of being a local 

 mutation from the common form of A. tridentata. 



The remaining forms of A. tridentata are all of small size. It is apparent that grazing 

 and burning have reacted upon the sagebrush itself to produce a series of new dwarf 

 forms in various stages of fixation. It appears certain that these forms have arisen in 

 more than one region, and hence constitute examples of polygenetic origin. Similar 



Fio. 21. 

 Unusual leaves of Artemisia tridentata tvpiea, all 

 from sterile shoots of a single plant otherwise typ- 

 ical. Leevining Grade, Mono County, California, 

 2,600 m. altitude, September 15, 1921, Clemmtt 

 (UC). AU X 0.8. 



forms have been produced where the sagebrush entered rocky, sterile, or subalpine areas, 

 as in the case of trifida, arbuscula, nova, and rothrocki. The distribution of these forms 

 is likewise such as to furnish further evidence of the action of polygenesis. This is 

 particularly true of rothrocki, which occurs only in Sierran and Rocky Mountain stations 

 750 miles apart. 



Of all these dwarfed subspecies, nova is perhaps the one which most closely approaches 

 typica. Its narrow inflorescence is very striking in the extreme form and is commonly 

 associated with low stature. This combination, to which is to be added the small number 

 of flowers developed in each head, suggests that the normal nutrition of the plant may be 

 interfered with. This subspecies, whether fixed in its characters or not, seems to be the 

 result of malnutrition, as is indicated by the fact that it usually and perhaps always 

 grows where the soil is shallow, stony, or otherwise unfavorable to full development. 

 Between Strevell and Albion, in extreme southern Idaho, the plants of nova are not 

 only low and with narrow inflorescences, but they are also quite black as contrasted with 

 typica. It here alternates and mixes with typica on apparently uniform soil and does 

 not intergrade with it. This suggests that it may be a more palatable strain kept low 

 by grazing. In other places the subspecies typica itself becomes much dwarfed, espe- 

 cially when competing with grasses, but without assuming the slender habit and narrow 

 inflorescence of nova. At other times it takes on all of the characters of nova, except 

 that the involucres remain canescent (see minor variation 11). This nicely indicates 

 the lack of unison in the variation of the characters used to separate the two forms. 

 The connection between typica and nova may be indicated by citing a series of specimens 

 with the former at one end, and the latter at the other, but with each of the series 

 differing from its neighbor by only a single trivial character. This series, easily duplicated 



