174 GENUS CHRYSOTHAMNUS. 



Soda Springs, Esmeralda County, Nevada, where the ranges of consimilis and viridulus 

 meet. Entirely different from the characters just considered are those which are plainly 

 modified by changes in the environment. These have to do with habit, size, and direc- 

 tion of leaf, relative amount (but not nature) of pubescence, leafiness, size of inflores- 

 cence, etc. Such characters often can be readily accounted for when the habitat is 

 examined. The ecologic origin of the forms is further suggested by their occurrence at 

 numerous localities and without regard to geographic isolation, but mutations may also 

 occur in this manner. 



The extent of variation in one floral character in plants of a single subspecies and 

 apparently of uniform germinal constitution is indicated in table 18. 



The figures used in table 18 are in each case the average of several measurements. 

 Similar studies of specimens at other localities and for oiher subspecies indicate an 

 equally great variability. The conclusion is that all of the more common subspecies 

 of C. nauseosus are in a highly plastic condition and that there exists even within a single 

 colony a large number of minor forms. Whether this is due to the presence of numerous 

 genetic strains or biotypes, or whether it is the result of fluctuating variability is not 

 within the province of this paper to decide. In either case, it indicates that "improve- 

 ment" in any desired direction may be brought about by selection, or by hybridization, 

 or by both of these methods. In this connection it may be pointed out that the percent- 

 age content of rubber in subspecies of C. nauseosus varies for different plants in a single 

 locality in much the same manner as the index characters used in the above table. This 

 is shown in a table of 180 chemical analyses recently reported upon by Hall and Good- 

 speed (Univ. Calif. Publ. Botany 7:227 to 233, 1919). 



GENERIC DIAGNOSIS. 



CHRYSOTHAMNUS Nuttall, Trans. Am. Phil. Soc. 11, 7:323, 1840. 



Branching shrubs and half shrubs. Roots fibrous, either with or without a taproot. 

 Stems erect or ascending, never twining. Herbage glabrous to tomentose, commonly 

 resinous and aromatic, impressed-punctate in two species. Leaves alternate, with- 

 out distinction between blade and petiole, entire or only scabro-serrulate. Heads in 

 cymes, thyrses, spikes, racemes, or panicles. Involucre cylindraceous ; bracts mostly 

 carinate or with a strong midrib, well imbricated in more or less distinct vertical ranks, 

 chartaceous or coriaceous, only the tips sometimes herbaceous. Ray-flowers entirely 

 wanting. Disk-flowers 4 to 20. Corolla tubular-funnelform, the tube passing gradu- 

 ally into the throat, 5-toothed or 5-cleft. Style-appendages exserted from the corolla- 

 tube, lanceolate-attenuate to filiform or subulate, either longer or moderately shorter 

 than the stigmatic portion. Achenes slender, terete or slightly angled or flattened, 

 rarely striate, glabrous to densely pubescent. Pappus copious, soft, dull white to reddish. 

 (Bigelovia, section Spuriae De Candolle, Prodr. 5:329, 1836. Bigelovia, sections Chryso- 

 thamnopsis and Chrysothamnus Gray, Proc. Am. Acad. 8:641, 642, 1873, and Syn. Fl. 

 12:136, 137, 1884.) 



According to Nuttall, the genus was named Chrysothamnus from its affinity to Chryso- 

 coma and from its brilliant golden-yellow flowers. A literal translation is "golden 

 wood," or "golden bush." The latter is used to some extent in the Rocky Mountain 

 States as a common name for C. nauseosus. This species as well as a number of others is 

 more commonly called rabbit-brush, which was given because of the utilization of the 

 thickets by rabbits as places of refuge. They sometimes eat the young shoots, but this 

 is not a universal habit. (For a discussion of common and Indian names see Univ. Calif. 

 Publ. Bot. 7:190, 1919.) 



