Rvdberg : Rocky Mountain flora 545 



a good many forms which I regard as hybrids. The two species 

 which seem to have produced the most hybrids are Card u its 

 americanus (A. Gray) Greene (not Rydb.) and C. griseus Rydb. 

 The former of these is comparatively common in Colorado, but 

 the latter is rather rare. Several of the specimens cited under 

 the latter in my Flora of Colorado do not belong to it, but are 

 hybrids of Cardans americanus and various species. The original 

 of C. griseus and later specimens collected by Osterhout do not 

 have the bracts dilated at all orerose; the spines of the involucral 

 bracts are long and somewhat flattened, and the leaves are darker 

 and less deeply divided than in C. americanus. The following 

 probable hybrids have been recognized, but, like Mr. E. P. Bick- 

 nell, in the matter of Rubus hybrids,* we wish " to divest the sub- 

 ject from all nomenclatorial claims" and "to be understood 

 merely as pointing out the probability of the occurrence of the 

 hybrids mentioned." 



Carduus americanus X griseus 



This has the leaves of C. griseus, i. e. dark green above, grayish- 

 tomentose beneath and with short lobes, as well as the strong and 

 broad spines of the involucral bracts of that species, and some of 

 the outer bracts are spinulose-ciliate; but most of the bracts are 

 erose on the margins and the inner ones have dilated tips as in 

 C. americanus. The following specimens are to be referred here: 



Colorado: Toland, Gilpin Co., July 20, 1906, Osterhout 3266; 

 Ward, Boulder Co., July 17, 1901, Osterhout 242Q. 



The former of these was labeled by Osterhout Carduus erosus 

 Rydb. (?) . The original C. erosus is quite different. To strengthen 

 the probability of hybridity, it may be mentioned that Mr. Oster- 

 hout has sent in specimens of one of the supposed parents, viz., C. 

 griseus, also from Toland, Gilpin Co., collected on the same date, 

 his 3267, the next number, and that C. americanus is a rather 

 common plant in Colorado. 



The latter of the two specimens was determined by me as 

 C. griseus, although I now regard it as a hybrid of that species 

 and C. americanus. C. americanus has been collected at Ward, 

 by Tweedy. 



* Bull. Torrey Club 37: 399. 1910. 



