CRITERIA. 



37 



so far as the species as a whole are concerned, as is indicated by the fact that although 

 the average number of flowers in the head is nnuch less in Seriphidium than in any other 

 section, the species are abundant in individuals and widely distributed. An intermediate 

 stage between Abrotanum and Seriphidium is indicated by A. bigelovi, a low shrub with 

 the aspect of certain forms of A. tridentata. Here the ray-flowers, recognized by their 

 peculiar 2-toothed corollas, are usually present, although reduced in number to only 

 one or two, but occasionally entirely suppressed, the head then consisting of only two 

 or three flowers with regular 5-toothed corollas. Perhaps this species represents the 

 beginning of the Seriphidium line, where the evolution of homogamous from hetero- 

 gamous heads is still in progress. 



In accordance with this discussion of the relationships of the various sections of 

 Artemisia to one another, the sequence in the following treatment conforms to the 

 diagram and key here presented, and hence departs quite radically from that found in 

 any preceding monograph. While this may at first seem somewhat confusing, it is 

 believed that the phylogenetic basis on which it rests is ample justification for its 

 adoption. 



CRITERIA FOR THE RECOGNITION OF SPECIES AND SUBSPECIES. 



It is obvious that the best criteria are those which have to do with the structure and 

 development of the flowers and particularly those of the fruits. Such differences in 

 character are not wanting in Artemisia, but they are extremely few. It is not surprising, 

 therefore, to find that these are of such importance that they are used as a basis for the 

 sections of the genus. Thus, the suppression of the ray-flowers in some cases and the 

 complete disappearance of the disk-achenes in others are both recognized as sectional 

 characters, as already discussed in the preceding chapter. Other characters, such as 

 minor modifications in floral structure, and especially that of the involucre and the more 

 vegetative parts of the plant, will now be taken up in detail. 



Fig. 2. — Achenes of Artemiria: a, b, from a single plant of A. tridentata typica (73923 UC); c, d, from another plant of A. 

 tridentata typica (71705 UC); e, /, from a single plant of A. cana (Hall 1167S); g, h, from another plant of A. 

 cana (Hall 11690). All of the achenes are more or less granular-glanduliferous. All X 16. 



Achene. — The shape of the achene in Artemisia apparently is a modification of the 

 prism. In some species it is still essentially prismatic, with four faces, but there always 

 is a more or less evident narrowing toward the base and the summit is truly truncate 

 in only a few cases. The nearest approach to the prism is seen in the achenes of some of 

 the subspecies of A. norvegica, A. tridentata, and A. cana, as shown on the respective 

 plates. The variation within a species, and even on a single plant, is greater than would 

 be expected. This variation is seen especially in the extent of incurving at the summit 

 and in the prominence of the angles, these latter often being reduced to ribs or nerves, 

 or in some cases they are quite obsolete. Something of this is shown in the accompany- 

 ing figures, where it will be seen that very unlike achenes occur in some of the species. 



