li)4 ( iiir<rsilii of ('(tl ifufiiid /' iihl iciit talis iit llottnni |\<ii.. 7 



siicli ;is riridiiliis and itnisimilis. at about ('i<jlit Ni-ai's. Tliis is for 

 plants w iMoliiii;^- five of six pounds and ^i-owin^ in oi'dinai'v alkaliiu* 

 soil \villn>n1 at1(nlion or dist urliancr liy man. Plants seldom reach 

 full size lici'oic tlicy ai-c five ycai-s old and tlicy often increase in 

 weiji'lit up to tru xcai's and p'M'lia|)s lonti'ci'. 



VI. I)lSTKl]^>rT10X AND HABITATS OF THE \AH10US 



FORMS 



Rubbef pi'odiK'iiig vai-ieties of wild Rabbit-])rusli are widely dis- 

 ffibuted in western North Amei'iea. They belon*"' to the Lower and 

 I'ppi'i- Sonoi'an life zones with slight extensions into the Transition, 

 and ran<ie in altitnde from about sea level in some of the desert basins, 

 to 8000 feet in the southern Colorado mountains. Some varieties of 

 ('Jirysoiliamiius occur at even higher altitudes but they have not been 

 examined as to their I'ubber content. The ])lants are most abundant 

 and of maxiiinnn size in the Great l^asin area. l)eeoming moi'e and more 

 scattered and aji]>arently diminishing in tlieii' percentage of rubber 

 as we pass outwai'd from this center of disti'ibution. The northern 

 limits are reached in Britisli Tohnnbia, Alberta, and Saskatchewan, 

 but no detailed studies have been made of ])lants from those districts. 

 They probably contain rubber in only limited (piantity and the areas 

 covered by the shrub are not extensive. Oui* northern tier of .states 

 supports some Chrijsothamnus but only in the more arid portions such 

 as southeastei'ii Washington and southern Idaho aiul Montana. The 

 most northerly points from which we have taken sami)les for analysis 

 are in eastern Oregon (Redmond, Burns, ete.^. eastern Washington 

 (Spokane), southern Idaho, and southern Wyoming (Rawlins, Lara- 

 mie). The best sami)les carried only -5 \wv cent of I'ubber and most of 

 them ran less than two jx'i- cent. This low yield uuiy be due to the 

 habitat oi- it niay be that a larger series of examinations would show 

 that we happened to select oidy low-gr;nle indixidu.ds and that the 

 average is really as high as in districts lyijig farthei- to the south. 



The eastei'Iy limits of the geinis are reached in South Dakota and 

 western Nebraska : the southerly limits in western Texas, southern New 

 Mexico, and southeni Arizona, with some ])ossible extensions into 

 Mexico, or at least into Lower California. Analyses have not been 

 made of plants fi-om these outlying (listi"icts but the indications are 

 that they ai'e not good rul)l)ci' |)i-o(lueers. On the Pacific Coa.st we find 



