CRITERIA. 43 



Roots. — The true roots, as contrasted with the rootstocks and caudex mentioned 

 above, have not been used for purposes of classification. The root system of A. filifolia 

 has been described and mapped by Weaver (Carnegie Inst. Wash. Pub. 286:73, fig. 25, 

 1919). 



Pubescence. — As in other botanical groups, this character is a misleading one to use, 

 if only its relative abundance and the accompanying effect on the general appearance of 

 the plant are taken into account. Thus, all gradations in the amount of pubescence on 

 different parts of the plant, and consequently in the appearance of the plant itself, are 

 well known in the numerous variads of A. vulgaris. More important is the persistence 

 of the pubescence when this is present. For example, the behavior of tomentum in 

 the subspecies of vulgaris is exceedingly variable, and yet certain ones exhibit a strong 

 tendency to retain the tomentum, while others have an equally strong tendency to drop 

 it as the parts mature. Even more important is the nature of pubescence itself. This 

 difference in the character of the trichomes and the direction they take in regard to the 

 surface of the epidermis is especially helpful in separating norvegica and its associates 

 from the vulgaris group of species. In the former the pubescence, when present, is 

 spreading, or villous, while in the latter it is more or less appressed, tangled, and therefore 

 tomentose. Yet even here it is possible to select forms of the two different groups that 

 are so much alike in this character that their separation becomes exceedingly perplexing. 



The pubescence of the receptacle is a matter of great importance. It occurs only in 

 one section of the genus, namely. Absinthium. This criterion separates a natural group 

 of the species, although it is probably not of sufficient importance to be rated as a 

 sectional character. The fact that the pubescence is easily determined, and that in 

 this section it is very long and copious, while it is entirely wanting throughout the rest 

 of the genus, leads to the retention of Absinthium largely because of practical consid- 

 erations. The corollas and achenes are all long-pubescent in spinescens, and this 

 character here seems to attain specific importance. However, in one subspecies of A. 

 tridentata, namely, parishi, the achene is long-pubescent, although no other characters 

 can be found which run parallel with this one. A suggested explanation is that parishi 

 has arisen through mutation in this single character. Aside from the rather pronounced 

 pubescence just under consideration, there are sometimes a few prominent hairs on the 

 upper part of the corolla. These are frequently described in species of the Old World, 

 but their presence or absence seldom runs parallel with other characters. Such hairs 

 are occasionally found in American plants, especially in forms of A. norvegica and in 

 A. patter soni. They furnish a nearly but not quite constant criterion for the recognition 

 of the latter species as distinct from A. scopulorum. In A. norvegica saxatilis the corolla- 

 tube is perhaps always long-hairy. 



Resins, oils, and odors. — Resins are not copiously developed in Artemisia and no 

 species is marked by impressed resin-dots of the foliage. There is, however, a prominent 

 exudation of resin on some parts, particularly on the flowers and achenes. These 

 resin-dots harden and become somewhat granular. Since they are more noticeable in 

 some species than in others, it may be that their presence affords contributor^' evidence 

 as to the standing of certain forms, but their use would require much more detailed studies 

 than have as yet jaeen given to them. The oils are abundant in some species and give 

 quite characteristic odors to the herbage in these cases. For this reason, the odor of 

 the herbage in each of the species is mentioned in the present monograph, although it 

 is recognized that such descriptions do not carry a clear conception of actual conditions. 

 It is only by extended observation in the field that one can come to recognize the different 

 sorts by odor alone. 



Habit. — All degrees of duration are found in the genus, from annual herbs to arbores- 

 cent, tree-like perennials. The annual and biennial plants have a definite taproot, but 



