ORIGIN AND DEVELOPMENT OF THE SECTIONS. 163 



ORIGIN AND DEVELOPMENT OF THE SECTIONS. 



The relationships between the various species will be taken up in detail in connection 

 with the descriptive accounts of each, as will also the relationships between the sub- 

 species. However, it seems desirable here to sketch the lines of evolutionary develop- 

 ment, at least as far as this concerns the principal species and the sections, and to present 

 the results in diagrammatic form. Although the general laws of phylogeny, as far as 

 they are understood, form the basis of this discussion, it does not follow that the results 

 represent more than the probable relationships. It is believed, however, that this 

 attempt to throw the species into natural evolutionary groups will be of service, espe- 

 cially if the doubtful cases are not definitely placed until more is known concerning them. 



It has been shown that the contact of Chrysothamnus with its most closely related 

 genus, that is, Haplopappus, is apparently best represented by the Punctati. This does 

 not necessitate the assumption that the other sections have arisen through this one. 

 Such a hypothesis would lead to the conclusion that the character of impressed-punctate 

 foliage was once developed and then lost. While this is possible, it seems improbable. 

 More logical is the assumption that the Chrysothamnus stock was developed from an 

 ancestral group close to Haplopappus section Ericameria but without the resin-dots of 

 that group. The development first concerned itself with modifications of the involucre 

 until this structure, or rather this assemblage of structures, was different from anything 

 in Ericameria, especially in its subcylindric shape and in the arrangement of the bracts. 

 It was perhaps from such a group that the Punctati were derived, their vegetative changes, 

 especially the formation of resin-dots, paralleling to some extent those going on in Eri- 

 cameria. In some such manner were these two sections evolved, sections so alike in 

 superficial appearance as often to be confused with each other, yet so unlike in origin, 

 indicated by difference in fundamental characters, as now to be assigned to different 

 genera. This is apparently a case of parallel variation in two groups not widely sepa- 

 rated phylogenetically. The connection with Ericameria is best seen by comparing 

 Chrysothamnus paniculatus with Ericameria diffusa. The Punctati are only two in 

 number, both shrubs of the Southwest and very closely related to each other. 



Without running counter to the laws of phylogeny, it may next be assumed that the 

 hypothetical group already mentioned gave rise somewhat farther up to the Typici. It 

 seems reasonable that this section is more primitive than any of the three remaining 

 ones, in view of the fact that each of these exhibits some striking peculiarity not pos- 

 sessed by the Typici and indicative in each case of a higher order of development. Tak- 

 ing then the Typici as the central section, we find its most characteristic representative 

 to be C. viscidiflorus. This is a very widely distributed species, which has itself under- 

 gone modifications in several directions, as is indicated by the nine subspecies and numer- 

 ous forms discussed in detail farther on. But aside from its success as a biologic type, it 

 possesses no characters indicative of an extreme evolutionary development. The in- 

 volucre gives positive evidence of vertical rows in the arrangement of its bracts, but these 

 rows are not very sharply defined; the twigs are devoid of any highly specialized pubes- 

 cence; and neither bracts nor style-appendages are unusually elongated. It has a very 

 intimate relative in C. greenei, but the other three members of the section, gramineus, 

 vaseyi, and albidus, are less closely connected. 



The Pulchelli are so much like the Typici that there can be no doubt as to their close 

 phylogenetic origin. The habit, the nature of the wood and bark, the type of inflores- 

 cence, and the relative length of appendage and stigmatic portion of the style-branch 

 are almost identical. But in the Pulchelli the carination of the bracts and their vertical 

 arrangement is carried to an extreme not otherwise known in the genus, and the long, 

 glabrous but not striate achenes are very different from those of the Typici. The two 

 species comprising this section are very closely related to each other. 



