HISTORY, LIMITS, AND RELATIONSHIPS. 159 



phora and a few incidental remarks, no discussion of relationships was entered into and 

 no differences were pointed out between Chrysothamnus and the several species of Haplo- 

 pappus, with which it seems to come into very close phylogenetic connection. 



While the relation of Chrysothamnus to Bigelovia has received much attention, its pos- 

 sible connection with Haplopappus has been scarcely more than suggested, and this 

 mostly through the transfer of species from one to the other genus without critical com- 

 ment. This is unfortunate, for the origin of the one has presumably been through the 

 other, that is, Haplopappus probably includes the nearest representatives of the ancestral 

 type of Chrysothamnus. Indeed, the two are so close at some points that, if it were not 

 for the almost universal recognition of the latter during the last twenty-five years under 

 one name or another, their complete union into one genus might be seriously considered. 

 It seems unwise, however, to disturb generic lines as long as the present arrangement 

 does no violence to the facts. Any other course would open the way for innumerable 

 generic combinations and segregations, each based upon individual judgment. 



Although the two genera now under consideration closely approach each other in some 

 respects, there is nevertheless a good basis, in addition to usage, for recognizing them 

 as distinct. Chrysothamnus differs from all species of Haplopappus in its consistently 

 narrower heads and, what is of greater importance, a decided tendency of the bracts of 

 the involucre to fall into vertical rows. The difference between this arrangement of the 

 bracts and the regularly imbricate arrangement encountered in the latter genus is per- 

 haps comparable to the difference between opposite and alternate leaves, but the bracts 

 are the modified leaves of a highly specialized structure, the involucre, and hence any 

 variation in their relative positions is of profound significance. In some species, such as 

 C. pulchellus, the vertical rows are very distinct, in others, especially among the Parryani, 

 the alignment is sometimes quite obscure. It is believed, however, that this is of signifi- 

 cance as indicating an ancestral trait, even when there is scarcely more than the tend- 

 ency left. In fact, it would be quite remarkable if the sharp vertical arrangement should 

 be strictly adhered to in any group of forms as large as this, when the various influences 

 that affect growth and development are taken into account. Another distinguishing 

 character, doubtless associated with the vertical arrangement of the bracts, is the 

 usually well-developed keel of these structures. In the more typical species the bracts 

 are rather sharply folded longitudinally along the midrib, which is thus emphasized to 

 form a distinctly sharp edge or keel. This is most noticeable in those species in which 

 the vertical rows are also plainly marked, while it may be quite obscure when the rows 

 themselves are not easily made out. 



Chrysothamnus is thus seen to differ from Haplopappus, its nearest relative, in the 

 narrow, cylindraceous heads, in the vertical arrangement of the involucral bracts, and 

 in the more strongly developed midrib of these bracts. However, in all of these char- 

 acters there is a shading-off in certain species, so that the two genera are not sharply 

 defined from each other. The contact between them is perhaps closest through the sec- 

 tion Ericameria of Haplopappus, of which certain members, such as H. brachylepis and 

 Ericameria diffusa, approach species of Chrysothamnus section Punctati very closely. 

 These two groups meet in their geographic distribution, possess a similar habit, and in 

 both cases the foliage is marked with impressed resin-dots. This easily determined pres- 

 ence of resin-dots has, in fact, led to the union of the Punctati with Ericameria (that is, 

 with Haplopappus section Ericameria) in most accounts, but this strictly vegetative 

 character, which has probably been independently developed in the two groups and is 

 associated with their xerophytic habitat, can not be considered as of importance when 

 compared with the shape of the heads and the shape and arrangement of the involu- 

 cral bracts, by which characters the Punctati are plainly to be associated with Chryso- 

 thamnus. This section is closely related also through C. alhidus, which, although of 



