40 GENUS ARTEMISIA. 



in A . bigelovi, a species which has ray-flowers, but these are very few in number and even 

 entirely wanting in some heads, while in all of its other characters it is similar to the 

 uniformly homogamous A. tridentata. 



The number of ray-flowers and of disk-flowers in a head is often of specific value. 

 These numbers are therefore always stated in descriptions and are used to some extent 

 in keys. The extent of variation is again indicated in the tables. 



The abortion of pistils of the disk-flowers was also used by Besser as a basis for sec- 

 tional classification. This abortion consists in the almost complete reduction of the 

 ovary and consequently of the achene, and in a considerable modification of the style. 

 The stigma in these aborted pistils is of course functionless. All of the species showing 

 this remarkable modification were placed by Besser in his section Dracunculus, and there 

 is no apparent reason for assuming that the section is other than a natural one. The 

 character is obviously to be looked upon as one indicating a high order of development, 

 and Dracunculus is therefore accepted as one of the two most highly specialized sections 

 of the genus. 



Involucre. — The structure of the involucre and the bracts often furnishes important 

 specific criteria in the Compositae. In Artemisia, however, the involucre, while yielding 

 some characters of value, is on the whole quite disappointing. The size of the involucre 

 is used to a very considerable extent, but it should be understood that this is usually 

 associated with the number of flowers in the head. In stating the size it is found that 

 the height is much more useful than the breadth. This is simply because the height is 

 a dimension which can be measured with definiteness, whereas the breadth depends 

 not only upon the state of maturity, but, in the case of herbarium specimens, upon the 

 pressure which has been applied in their preparation. The largest heads in the species 

 found in America occur in A. stelleriana. The heads are here so large that the species 

 may always be distinguished by this character alone, except for an occasional very large 

 head of A. vulgaris tilesi. In A. vulgaris there is a very considerable range of size, as 

 indicated by the tables, but it is noteworthy that one subspecies, namely, litoralis, has 

 an involucre regularly so narrow that its dimensions never overlap those of any other. 

 The fluctuations both in height and breadth within subspecies and within species is 

 indicated in a number of the tables. 



The relative length of bracts within a single involucre has been used in many instances 

 by Rydberg in his treatment of the genus in the North American Flora. It is true that 

 the outer bracts are sometimes so differently set off from the inner that this character 

 of relative length becomes of importance, but it is impossible to detect the sharp differ- 

 ences that one is led to assume from his descriptions. The absence of differences where 

 they are supposed to exist is indicatedin thefigures on pages 124 and 149. The outer bracts 

 are usually shorter than the inner and sometimes they have their own peculiar shape, but 

 the gradation is so slight and the external bracts undergo so many modifications that 

 such criteria must be used with exceeding caution. Even the inner bracts are not con- 

 stant in the shape of the exposed portions. The number is sometimes helpful, and this 

 is brought out together with the extent of the variation in the tables, where actual 

 counts of large series are indicated. 



Inflorescence. — By far the most common type of inflorescence is the panicle. All 

 others are modifications of this. When the panicle is narrow and much simplified, it 

 may be either racemose or spicate. When it is much shortened and condensed the 

 panicle becomes capitate. These various forms of inflorescence are modifications of 

 the paniculate type, as seen from the frequent series of intergradations that can be 

 found almost anywhere. They are also derived forms as evidenced by the fact that 

 the panicle is by far the most abundant and that it occurs in forms which are primi- 

 tive in their other features. In only one case does the modified type of inflorescence run 



