C. NAUSE08US. 225 



it is better to fall back upon a distinction based upon the pubescence of the involucre. 

 This character runs approximately parallel with what are believed to be natural lines 

 and its use is thus very convenient, although, as is to be expected in a highly variable 

 species like C. nauseosus, some forms plainly belonging by all other criteria to one group 

 of subspecies will have a pubescence of the involucre which very closely approaches that 

 of the other group. In fact, if this criterion alone is used, it will sometimes lead to an 

 erroneous placing of a subspecies, especially if the pubescence is very scant or minute. 

 Naturally, the pubescence of the involucre in the typicus group is only an expression of 

 the general tendency toward an excess of pubescence in the whole plant. The herbage of 

 all of the members of this group, with only now and then an exceptional form, is quite 

 gray or sometimes even white as compared with the usually green or yellowish-green 

 herbage of the graveolens branch. The two can not be accepted as distinct species, as is 

 evidenced by the very nature of the character used for their detection and also by the 

 frequent intergrading forms just mentioned. The contact seems to be between speciosus 

 and graveolens, since forms are constantly recurring which can be about as satisfactorily 

 placed in one as in the other of these subspecies. It seems probable that speciosus and 

 graveolens come the nearest to representing the ancestral form in which the primary 

 cleavage took place. This hypothesis finds some substantiation in the facts of geo- 

 graphic distribution. The subspecies speciosus is most abundant across the northerly 

 part of the Great Basin, that is, west of the Rocky Mountains, while graveolens belongs 

 chiefly to the plains and valleys of the Rocky Mountain States, but extends northwest 

 to southern Idaho, thus overlapping the range of the former. Ecologically, speciosus 

 belongs to mildly alkaline soil and runs up on slopes where there is perhaps no alkali. 

 The other members of the typicus group are inhabitants of non-alkaline soils, except for 

 occasional forms (especially plattensis) which have become adapted to moderately saline 

 conditions. On the other hand, graveolens grows in soil quite strongly alkaline and 

 among the other subspecies of its group are some which run down into strongly impreg- 

 nated soils. 



In following the typicus branch, it is first noted that two of the subspecies, gnaphalodes 

 and hololeucus, differ so radically from the others that they are assigned to a group extra- 

 neous to the principal assemblage. They are very much like the others in superficial 

 appearance, but differ from all of them in the very short, erect teeth to the corolla and 

 especially in the comparatively short stylar appendages, although both of these struc- 

 tures are much shortened in occasional plants of other subspecies. By reference to table 

 23 it will be seen that in the 30 specimens examined the appendage is always shorter 

 than the stigmatic portion, whereas in the 138 specimens of other subspecies the append- 

 age is always longer than the stigma, except in a very few cases (see especially sub- 

 species typicus) . The measurements were made on specimens which had been previously 

 determined as gnaphalodes and hololeucus because of their other characters. The short 

 corolla-lobes furnish a useful criterion for the identification of these subspecies, but it is 

 not always strictly applicable, for, as will be seen by reference to the table, the shortest 

 lobes in others are not infrequently shorter than the longest ones in these. But the 

 general parallelism between these two characters, together with the peculiarly aromatic 

 odor of the herbage, and certain other considerations, leads to the conclusion that the 

 two subspecies under consideration are closely related to each other and that they con- 

 stitute an evolutionary group quite distinct from the others. Subspecies gnaphalodes is 

 abundant on well-drained soils from middle Nevada and western Arizona to the Sierra 

 Nevada, while hololeucus occurs only as scattered individuals or in small groups within 

 this same area. The plants of the latter are so white as compared with the former that 

 the two can be distinguished in the field without difficulty. The origin of hololetums by 

 mutation at different places and times seems quite probable. 



