GENUS ARTEMISIA. 

 RELATIONSHIPS AND GENERIC LIMITS. 



Artemisia is a member of the Anlhemideae or mayweed tribe of the Compositae. 

 Its nearest ally is Tanacetum, from which it differs by only one constant technical char- 

 acter, namely, the type of inflorescence, although several other characters are so nearly 

 constant as to remove any thought of uniting the two. 



The inflorescence of the heads in Artemisia is always of the paniculate type. Some- 

 times the panicle is reduced to a single raceme, but never or rarely to a single head, and 

 nowhere in the genus do we find any approach to a cymose inflorescence. Tanacetum, 

 on the other hand, has a distinctly cymose arrangement of the heads, and even when 

 the head is solitary, as sometimes occurs in T. huronense, T. bipinnatum, T. compactum, 

 and other species, this is evidently the result of reduction from the cymose type as 

 indicated in most cases by the presence of cymes in other individuals of the same species. 



A second character useful in the recognition of Artemisia as distinct from Tanacetum 

 is found in the shape of the anther-tips. In the former genus these are alwaj^s lanceolate 

 or subulate and pointed; in Tanacetum they are broadly lanceolate to ovate and more 

 or less obtuse, with the single exception of the recently discovered T. compactum, of 

 western Nevada, in which species the tips are subulate and acute. 



Although Artemisia and Tanacetum constitute two rather well-defined and natural 

 genera, various attempts have been made in the literature to establish additional ones 

 by the process of taxonomic segregation. One of the earliest of these attempts was by 

 Tournefort, who in 1700 recognized Artemisia, Abrotanum, and Absinthium (Inst. Rei 

 Herb. 1 : 457-460). These proposed genera were based in part upon floral characters 

 and hence they still serve as subgenera or sections in many modern treatments, but 

 their full generic recognition is without practical value, since each contains at least a 

 few species which, in all but purely technical characters, very closely resemble certain 

 species in one of the other groups. The other attempts to establish genera within 

 Artemisia and Tanacetum have been based upon characters of slight importance, and 

 therefore the generic standing of the resulting segregates is very insecure. The latest 

 treatment of the North American forms is by Rydberg (N. Am. Fl. 34 : 242-285, 1916). 

 Since this author is inclined to the acceptance of genera on less important characters 

 than those demanded by most botanists, it is not surprising to find that all of the earlier 

 proposed segregates, except those of Tournefort, are here treated as genera and three 

 new ones added. A list of these, with a tabulation of the principal characters used in 

 their differentiation is given in table 3, at bottom of next page. 



The following key has been prepared in order to bring out the contrasting characters 

 fully. It differs in some details from the key given by Rydberg. According to the 

 treatment preferred here, the first four "genera" fall into Tanacetum, the fifth stands 

 as Crossostephium but confined to a single Old World species, and the last two are referred 

 back to Artemisia. 



Inflorescence cymose, the cyme occasionally reduced to a single head. 

 Anther-tips lanceolate or ovate, more or less obtuse. 



Corollas of the marginal flowers oblique; pappus coroniform 1. Tanacetum. 



Corollas of marginal flowers not oblique; pappus none. 



Achenes swelling and becoming gelatinous in water; receptacle pubescent 2. Vesicarpa. 



Achenes neither swelling nor becoming gelatinous in water; receptacle glabrous. ... 3. Sphaeromeria. 



Anther-tips subulate, acute 4. Chamariemiaia. 



Inflorescence racemose-paniculate. 



Receptacle glabrous or pubescent but never chaffy-bracted. 



Achenes and corollas glabrous to arachnoid, but not villous. 



Pappus a lacerate crown; ray-flowers in 2 rows 5. Crossostephium. 



Pappus wanting; ray-flowers in 1 row or wanting 6. Artemisia. 



Achenes and corollas arachnoid-villous 7. Picrothamnus. 



Receptacle chaffy-bracted 8. Artemisiastrum. 



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