THE AMERICAN NATURALIST. [Vol. XXXVIII. 



Historically the desert regions of North America are of interest, 

 that area which was known as " the great American Desert " by car- 

 tographers as late as 1843, does not really exist as such. The des- 

 erts of this continent may be designated as the Sierra-Nevada desert, 

 comprising portions of Utah, Idaho, Washington, Oregon, Nevada, 

 California, Arizona, Baja California, Sonora, and Sinaloa ; and the 

 Chihuahua desert, which occupies the tableland of Mexico east of 

 the Sierra Madre and north into Texas, Arizona, and New Mexico. 

 It is further stated that for the purposes of this paper the desert 

 lands of the Dakotas, of Montana and Wyoming may be considered 

 as the extreme northern arm of the last named region. 



As the closing section of the paper there is a discussion of the 

 results of experiments by one of the authors, carried on at an earlier 

 date. A comparison of two desert types, Mentzelia pumila, and an 

 Artemisia, with two moisture loving forms, tomato and Eucalyptus 

 globosus, shows that even in their natural habitats, where the con- 

 ditions for transpiration are of course vastly in favor of the desert 

 plants, the disparity between the water evaporation of the two is 

 very great. As to temperature it was found that the plant-body of an 

 Opuntia gave a maximum in the forenoon of as much as 1 1 1.2° Fahr., 

 while in specimens of Cereus temperatures of 1 13° to 1 15° Fahr., were 

 frequently found. The fact that this is above the critical temperature 

 usually given for chlorophyll leads to the authors' suggestion that the 

 protoplasm and the chloroplasts may have undergone changes which 

 adapt them for such conditions, although, they add, it is not unlikely 

 that the death of plants in such regions is as often the result of too 

 great insolation as of lack of water. 



The authors make no claim that their narrative is an exhaustive 

 account of the regions visited, which considering the short time spent 

 would of course be quite impossible, but express a hope that it will 

 serve to show the great diversity which exists in the several floras 

 which go to make up our desert flora as a whole. In this they can 

 certainly feel that they have succeeded both in the text and in the 

 admirable illustrations which are profusely scattered through the 

 pamphlet. To any intending student of the conditions which exist 

 in our deserts it is bound to be of great service and the full bibli- 

 ography by W. A. Cannon which is appended will be an additional 

 aid. This is the first publication relative to the desert laboratory, we 

 may hope that many more will emanate from this source. 



H. M. R. 



{No. 447 was mailed May 3, 1904.) 



