62 University of California Puhlications in Botany [Vol. 9 



vast majority being in Carex (44 species), the largest genus of the 

 boreal flora. 



Of the 5 largest boreal plant families of the Sierra, counting 

 number of species, Scrophulari-aceae has the largest genus-species ratio 

 with 41 species divided among 8 genera, or slightly more than 5 species 

 per genus. Mimulus (13 species) is the largest genus with Pentstemon 

 (9 species) and Castilleja (8 species) next in order, the other 5 genera 

 present within our region dividing the remaining 11 species, 2 of the 5 

 being monotypic. 



Rosaceae in its 14 genera includes the same number of species 

 as Gramineae, i.e., 38. Potentilla is the largest genus, having 13 

 species; the nearly allied genus Ivesia takes 7 species, leaving 12 

 genera to include the remaining 18 species, though of this dozen genera 

 7 (more than half) are monotypic. 



These five families — Compositae, Cyperaceae, Scrophulariaeeae, 

 Gramineae, and Rosaceae — include 66 genera and 260 species, or 28.4 

 per cent of the genera of Angiosperms within our limits, and 43.1 

 per cent of the species. The reasons for this predominance of com- 

 posites, sedges, fig-worts, grasses, and rosaceous plants in the high 

 mountain region are still obscure, but the character of their repro- 

 ductive bodies may, in part, explain their successful occupation of 

 the alpine and subalpine countrj^ Excluding Scrophulariaeeae, the 

 typical fruit of these groups of plants is the achene, small, of light 

 weight, and well protected against injury by the hardened ovary wall 

 which encloses an embryo provided with stored food. The achene 

 is the exclusive type of fruit in Compositae, Cyperaceae, and Gram- 

 ineae (here slightly modified as the caryopsis or grain), while in 

 Rosaceae, it is the fruit of those genera, like Potentilla, Ivesia, and 

 Horkelia, which, if number of species be interpreted as a criterion of 

 generic success, have been most successful. In Scrophulariaeeae 

 achenes are replaced by many-seeded ovaries, the seeds themselves 

 being small and enclosing an embr^^o in copious albumen, the whole 

 seed analogous to an achene from the point of view of ease of distri- 

 bution and provision for germination. When one recalls the rigorous 

 features of the high mountain habitat, the temperature extremes, the 

 force of the wind, and the uneven nature of the terrane, the advan- 

 tages which the achene or some similar type of propagule may be 

 presumed to have seem significant. 



The other 57 per cent of the boreal angiosperm fiora is made up 

 of the remaining 46 families of flowering plants having representation 

 in our region. These families are unequally represented. 



