44 GENUS ARTEMISIA. 



the transition from annual to biennial habit is so often dependent upon ecologic con- 

 ditions that this character is of but slight importance. There are certain places in the 

 genus, however, where only annual and biennial species are found, while in other sections 

 plants of short duration are not known to occur. A great majority of the American 

 species are perennial herbs, growing either from rootstocks or from a caudex that sup- 

 ports a number of stems. The underground structures have not been examined in 

 detail and their characters have been used but slightly in classification. The caudex 

 habit is often an extreme form, derived from a rootstock, the stock becoming much 

 shortened and the stems growing in close proximity. It often happens that within a 

 single species some forms will have extensively creeping rootstocks, while others will 

 have a short caudex, the latter usually belonging to more northerly habitats. In almost 

 all cases when the plant is an herb the stem is simple up to the inflorescence. 



The shrubby habit has been developed at three different places in the genus, or at 

 least this follows if the present classification into sections is a natural one. However, 

 the largest shrubs, as well as the largest number of shrubby species, are found in the 

 section Seriphidium. These shrubs show considerable variation in habit, and hence 

 the differences are not sharp enough to permit of their use as specific criteria. The size 

 is sometimes helpful, but it is not always dependable. For example, the largest shrub 

 of all is A. tridentata, the most common of the sagebrushes, yet even in subspecies iypica 

 of this are found forms sometimes less than 1 dm. high. Such reduced forms are often 

 easily accounted for by field studies in that they are due to unfavorable environmental 

 conditions, usually to competition, especially with perennial grasses, or to continuous 

 browsing, to fire, to poor soil, or to a combination of all of these factors. One of the 

 American species of shrubs stands out very sharply from all the rest by its decidedly 

 spiny habit. This is A. spinescens, a species which is well set off also by important 

 floral characters. 



RELATIVE PLASTICITY OF THE SPECIES. 



The genus Artemisia includes species representing nearly all degrees of plasticity. 

 Some are in a state of constant and extensive variation, and by means of their numerous 

 forms are enabled to occupy very unlike habitats. Such species come naturally to have 

 a wide geographic distribution and also a wide altitudinal range in mountainous districts. 

 While their forms are difficult to treat taxonomically, because of this large number and 

 the striking dissimilarity of the extremes, the determination of the evolutionary lines 

 is rendered fairly certain because of the presence of intermediate forms and the compara- 

 tive ease of manipulation in experimental studies. The most notable species of this 

 class is A. vulgaris. This ranges almost throughout the temperate and subarctic portions 

 of the Northern Hemisphere and comprises an almost endless series of variations, some 

 65 of which have been described as distinct species by various authors. It seems quite 

 probable that some are now fixed in their principal characters and that they are limited 

 to particular habitats in consequence, but that many are still in a state of flux, at least 

 in some portion of their range. 



Other species which are represented by more than the average number of forms are 

 norvegica, campestris, dracunculus, and tridentata. These do not possess the high plas- 

 ticity of vulgaris, but each is represented by 15 or more named variads in addition to 

 many minor forms. All are of wide distribution and have also a large altitudinal range. 



Of the less plastic species, only A. frigida is of wide distribution. It has but few 

 named variations in America, but a considerable number of them in Asia. A few intro- 

 duced species are not here taken into account, since in each case the American plants 

 have in all probability descended from one genetic strain. The remaining species, such 

 as spinescens, parryi, franseriaides, pedatifida, etc., are all rare or local, or confined to 



