2 



The factors affecting plant distribution on San Jacinto Mountain 34 



Altitude, temperature, and moisture 34 



Slope exposure 34 



Desert winds 38 



Rock surface, avalanches, and land slides 40 



Water courses 41 



The battle of the factors 42 



A comparison of montane and desert conditions 44 



IV.— Origin and Affinities of the San Jacinto Flora. 



The boreal element , 47 



The austral element 50 



The presence of Arizonan and New Mexican species 50 



V. — Catalugue of Species Collected in or above the Yellow Pine 



Belt 51 



VI.— Index 133 



LIST OF PLATES. 



1. Map of San Jacinto Mountain. 



2. Map of Life Zones and Timber Belts. 



3. Lower Edge of Coniferous Forest. 



4. Coniferous Forest; Strawberry Valley. 



5. A Forest Island; Tahquitz Valley. 



6. Summit of San Jacinto Mountain. 



7. Chalk Hill and Marion Peak. 



8. Sonoran Islands near Chalk Hill. 



9. Rock Surface and Forests; Strawberry Valley. 



10. MonardeJla macrantha arida. 



11. Monardella macrantha tenuiflora. 



12. Monardella macrantha pinetorum. 



13. Leaf sections of Monardella. 



14. Oxytheca emarginuta. 



