66 • University of California Publications in Botany L^ol. 9 



In western North America there are recognized, by very general 

 agreement, at least three fairly distinct divisions : the Rocky Moun- 

 tains, the Pacific Northwest (Oregon, "Washington, Idaho, and British 

 Columbia), and the Californian region. Dr. Rydberg has shown that 

 the Rocky Mountains should not be considered as a unit floristically, 

 but should be divided into the Northern Rockies and Southern Rockies 

 with the subalpine plains of southern Wyoming intervening.* It is of 

 some interest to note the relative numbers of plant forms possessed in 

 common by the Sierra Nevada and by each of these subdivisions of the 

 Rocky Mountains. The geographic analysis of the Sierran high moun- 

 tain flora presented above indicates that of the 682 plants distin- 

 guished in this report: 



261 (a plus g plus /it) are common to the Sierra Nevada and the 

 Southern Rockies. 



286 (a plus d plus h) are conunon to the Sierra Nevada and the 

 Northern Rockies. 



361 (a plus c plus d plus j) are common to the Sierra Nevada and 

 the Pacific Northwest. 



Expressing these similarities in familiar terms, we find that over 

 one-half of the plants distinguished in the Sierra Nevada are also 

 present in the Northwest; more than one-third are conunon to the 

 Sierra and to the Southern Rockies ; an intermediate fraction expresses 

 the degree of similarity with respect to the Northern Rockies. 



KEY TO THE FAMILIES OF THE ANNOTATED LIST 



Sporophylls free or, if aggregated, never forming true flowers; plants with 



spores and without seeds. PTERIDOPHYTA (Ferns and Fern Allies) 



Leaves usually ample or, if small, never reduced to scales forming sheaths 



about the solid stems; sporophylls not aggregated into terminal cones 



(stroiili) formed of peltate scales. 



Terrestrial plants; leaves never grass-like. 



Leaves (fronds) not closely overlapping or imbricated, of medium to 

 large size, each with a stalk or stipe; sporangia numerous on each 

 sporophyll. 



* Torreya, vol. 12, pp. 73-85. 



t 1i includes a number of plants, some of which are common to the Sierra and 

 both Northern and Southern Eockies, and some common to the Sierra and only 

 one of these subdivisions, but at present the details of plant distribution for 

 eastern Idaho, southwest Wyoming, and western Colorado, are too imperfectly 

 known, at least to me, to warrant an attempt to distinguish the exact ranges of 

 the plants included in this group. Accordingly h is added to both of the totals 

 assigned to the divisions of the Eocky Mountains, a procedure not affecting the 

 degree of similarity with the Sierran flora of tliese two divisions as compared to 

 each other, though it may affect the value of comparing either of these totals 

 with that assigned to the Pacific Northwest. 



