CRITERIA. 171 



Pubescence in Chrysothamnus, aside from the special type of tomentum just described, 

 is scarcely noticeable, the twigs and foliage being never more than puberulent. In the 

 Punctati and in certain species of other sections, it is entirely wanting. Sometimes, as 

 among the subspecies of viscidiflorus, it will occur as a fairly den.se though minute puberu- 

 lence in certain plants, while others almost exactly duplicating these in every other respect 

 will be perfectly glabrous. This is so frequent that glabrous and puberulent forms may 

 be expected of any variad in that species. Their possible origin through mutation is 

 suggested in discussing the relationships of C. viscidiflorus. When such forms are quite 

 marked and of frequent occurrence, and especially if they have been given specific rank 

 elsewhere, they are treated in this paper as subspecies; when they appear only occasion- 

 ally, they are mentioned only in the list of minor variations. To assign specific or even 

 subspecific rank in all such cases would so enlarge and complicate taxonomic monographs 

 that their usefulness would be seriously impaired. It is very evident, however, that a 

 complete systematic treatment will need to take account of all such variations. 



Resins, oils, and other physiological products. — Throughout the genus there is a notable 

 tendency to the formation of resins and oils. This is associated with the xerophytic 

 habitat of the plants. The resins are not exuded through the trichomes, but directly 

 through the surface of the epidermis proper. The herbage is therefore often glutinous 

 or viscid. This character is more marked in some species than in others without, how- 

 ever, furnishing a useful criterion for distinguishing them. The reason is that the differ- 

 ence is entirely relative and dependent quite as much upon local conditions or even time 

 of day as upon an inherent quality. Much more important is the presence of definite 

 pits, or wells, into which the resin exudes, forming the resin-dots of the Punctati. These 

 are structural and therefore of greater significance than resinous or viscid coatings. 

 Since they are easily recognized their presence furnishes an ideal specific character. 



The odor of the herbage is an aid to one familiar with the plants. Its diagnostic value 

 is not great because of the absence of standards for comparison. The Punctati possess 

 a peculiar and very pleasing odor whereby one may recognize a plant as belonging to this 

 section, even though the flowers are wanting. In C. nauseosus the odors, probably due 

 to the presence of essential oils, vary somewhat in unison with other characters, so that 

 they are useful in a few cases for the recognition of subspecies. Thus the subspecies 

 viridulus emits a very strong, disagreeable odor when the twigs are broken or when 

 burned. A very similar though less pronounced odor is noticeable in the closely related 

 subspecies consimilis, pinif alius, and graveolens. On the other hand, the herbage of sub- 

 species gnaphalodes, and especially of subspecies hololeucus, usually has an exceedingly 

 pleasant fragrance, "suggesting a combination of tropical fruits and berries." Under 

 favorable conditions these forms may be recognized by the odor alone. 



Habit. — All of the species of Chrysothamnus are shrubs. In one case, namely C. gram- 

 ineus, the shrubby nature is not pronounced, although the base of the stem is perennial 

 and decidedly woody. In other species the plants are sometimes quite short, but they 

 are always woody and partake of the habit of shrubs; plainly they are reduced or dwarf 

 forms closely related to the more shrubby types. The transition between these dwarf 

 shrubs, some of which are only 1 dm. high, to the arborescent forms, which sometimes 

 attain a height of 24 dm., is so gradual as to preclude the use of this character for dis- 

 tinguishing categories above the rank of subspecies. 



