164 GENUS CHRYSOTHAMNUS. 



In order to reach the Nauseosi it is now necessary to leave the line which culminates 

 in the highly modified PulcheUi and pick up the evolutionary thread somewhere below 

 the Typici. This is necessitated by the appearance in the Nauseosi of a seemingly super- 

 ficial but really very important character, namely, the remarkably pannose tomentum 

 of the twigs. To one familiar with this matted, felt-like covering so closely applied to the 

 bark, it represents a character of greater importance than is usually found in the pubes- 

 cence. Since this indicates an evolutionary stage not reached by the PulcheUi, and since . 

 these on the other hand present highly specialized characters not found in the Nauseosi, 

 the conclusion follows that neither has been derived from the other. The latter is there- 

 fore indicated as branching off from near the Typici. It includes C. pyramidalus, a little- 

 known Mexican species which separated from the original stock in early times, and two 

 others which are much better known. These are C. parryi, a polymorphous species with 

 its center of distribution in the Great Basin, and C. nauseosus, an abundant and widely 

 distributed species of western North America. Ten subspecies of the former and 20 of 

 the latter are recognized in the present treatment. 



CRITERIA FOR RECOGNITION OF SPECIES AND SURSPECIES. 



Achene. — Two very unlike types of achene are encountered in the genus, but it is easy 

 to see how one of these may have been derived from the other. In the common form it 

 is either obscurely 5-angled or terete and 5-nerved, always tapering slightly from summit 

 to base but not sufficiently so to be described as turbinate, and usually covered with a 

 dense pubescence. In the other form, which occurs only in C. vaseyi and C. gramineus, 

 the essentially glabrous achene is nearly terete or slightly flattened, tapers but slightly 

 toward the base, and is distinctly 10-striate. This type is so different from the extreme 

 of the other that the character might be considered as generic were it not for other fea- 

 tures, especially of vaseyi, which definitely relate these species to the other Chrysothamni. 

 The two are not so far apart as would at first seem, as is indicated by such achenes as 

 those of C. nauseosus leiospermus, which are glabrous, terete, and with 5 principal striae, 

 but often with an additional fainter nerve between each pair of the more prominent ones, 

 thus approaching the terete, 10-nerved type of C. gramineus. The pubescence, or its 

 absence, has been used as a specific character, even in cases where the achenes are other- 

 wise identical with those of other species. But it has been shown in a recent paper (Hall 

 in Univ. Calif. Publ. Bot. 7:173, 1919) that this character is a variable one and that 

 three species based largely upon it, bigelovi, glareosus, and leiospermus, are all subspecies 

 of C. nauseosus. In C. pulchellus the achenes are usually glabrous, but sometimes spar- 

 ingly pubescent. Although this variation has been noted in descriptions (for example, 

 Rydberg, Fl. Rocky Mts. 855, 1917), there has been no attempt to use it as a basis for a 

 new species. A tendency toward the loss of the character in a species in which the 

 achenes are almost always densely villous or strigose is seen in C. viscidiflorus, where a 

 form described as C. marianus Rydberg has achenes only sparsely strigose or at times 

 apparently almost glabrous. The length is a valuable feature, but one to be used with 

 caution, since in most herbarium specimens the achenes are not fully developed. 



Pappus. — The pappus is so nearly uniform in Chrysothaynnus that it has not proved 

 very helpful in supplying distinctions between species or subspecies. It is more rigid 

 in some than in others. Sometimes it changes more rapidly and decidedly with age to a 

 tawny hue or becomes ferruginous, but while such characters are of assistance, even 

 when variable, they are impossible of expression in quantitative terms and therefore 

 difficult of application. The length of the pappus is of more definite value, especially 

 when taken in comparison with the length of the corolla. 



Style-branches. — In Chrysothamnus and related genera much significance has been 

 attached to the shape and length of the style-branch and its appendage. While these 



