26 



Mexican white pine {P. Many trees conspicuously No cones apparent 



strobiformis) filled 



Douglas fir {^Pseiidotsuga Cones very abundant Cones scarce 



taxifolia) 

 White fir [Abies concolor) No cones found in either 



year 



In the fall of 1906, the first two severe frosts occurred in 

 the aforesaid mountains on the nights of October 22 and 23. 

 Practically all the herbaceous plants were killed to the ground, 

 A few days later the deciduous tree species along Riggs Creek 

 were examined. Jiiglans riipestris, Fraxiniis, and Vitis had all 

 their leaves killed, proving to be the ones most easily injured. 

 Schmaltzia was almost killed in this open canyon, yet in the nar- 

 row Bonita Canyon near by, which has high, perpendicular walls, 

 this genus was collected a week later, remarkably preserved. 

 Certain acacias behaved likewise. Sycamore did not suffer 

 greatly, but the least harm came to the willow and the cherry. 

 The leaves of the latter hang on the tree till very late in autumn, 

 hardly losing their green color before they fall. Of all the 

 deciduous arboreal species of this region, this approaches most 

 nearly the evergreen habit. Thus it is possibly relatively frost- 

 hardy for the same reasons as the very frost-resistant as well as 

 drouth-defiant evergreens, such as the oaks. On the whole it 

 appeared that the species which lived upon the least moisture 

 were also the most frost-hardy. 



On the other hand, as is well known, the presence of an 

 abundance of water has often a powerfully protective effect. 

 The snowberry {Symplwricarpus), growing at an altitude of 8,000 

 feet, had its leaves nearly killed where growing in the open, 

 severely bitten where standing under trees, damaged but little 

 where growing without a canopy but on springy soil, and escaped 

 without any harm whatever where it stood under the spreading 

 boughs of the white fir while the water trickled about its roots. 



Other shrubby genera occurring in this place are Optilaster, 

 Holodiscus, Rnbus, Salix, and Ptelea, but Syinplioricarpns appears 

 to be the hardiest of all. 



Desert Botanical Laboratory, 

 Tucson, Arizona. 



