RYDBERG: 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 Carduus 
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 Carduus americanus and various species. The original 
of C. griseus and later specimens collected by Osterhout do not 
have the bracts dilated at all or erose; 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 XGRISEUS 
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: 
CoLorapo: Toland, Gilpin Co., July 20, 1906, Osterhout 3266; 
Ward, Boulder Co., July 17, 1901, Osterhout 2429. __ 
The former of these was labeled by Osterhout Cardwus erosus 
Rydb. (2). 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. 
