DEPARTMENT OF BOTANICAL RESEARCH. 57 



Influence of Low Temperatures on Giant Cactus, by Dr. Forrest Shreve. 



Experimental evidence has been secured going to show that the giant 

 cactus (Carnegiea) is capable of withstanding below-freezing temperatures 

 for periods of i6 to 20 hours, but is not able to survive such exposure for 

 30 to 40 hours. In the desert valleys of southern Arizona freezing tempera- 

 tures never persist over an entire day, the longest single period of frost 

 recorded for Tucson being 19 hours. The distributional limit of the giant 

 cactus is so situated as to indicate that its inability to withstand a duration of 

 frost of 30 to 40 hours is the factor responsible for the limitation of its 

 northward extension and of its vertical range in the desert mountains. These 

 results suggest the importance of winter-cold factors in limiting the distribu- 

 tion of desert species on the slopes of the larger desert mountains and at the 

 more elevated or more northerly edges of the American Desert, and would 

 appear to account for the inability of some of the desert species to migrate 

 into regions of more favorable moisture conditions, out of which "compe- 

 tition of better adapted species" has been supposed to keep them. 



Evolution of the Chrysomelid Beetles of the Genus Leptinotarsa, by 

 Prof. W. L. Tower. 



The present report is a very brief resume of the chief operations and of 

 the results thus far obtained at the Desert Botanical Laboratory during the 

 seasons of 1910 and 191 1. Throughout the two years the conditions have 

 been, so far as these animals are concerned, unusually severe, even for these 

 desert areas, and the labor of maintaining the experiments has therefore 

 been greatly increased. Many new and minor problems have arisen, which 

 demand time and attention in order that the major objects of the investi- 

 gation might not suffer. The immediate care and oversight of the experi- 

 ments have been in the keeping of Mr. J. K. Breitenbecker during both years, 

 and to his enthusiastic aid much of our success is due. Many of the minor 

 problems that were presented at the beginning of the work at Tucson have 

 been eliminated, especially the question of supply of food ; no further diffi- 

 culty is anticipated in maintaining the necessary food supply with the mini- 

 mum expenditure of time and energy. The measurement of the conditions 

 under which the different experiments are carried on has consumed consider- 

 able time, and there are now available long series of determinations and 

 measurements of the natural conditions surrounding the cultures and of the 

 effect of the cages in damping or intensifying any of the factors of the natu- 

 ral environment. An unusually complete picture of the physical conditions 

 in these experiments is now available. It is usual in work of this kind to 

 employ the records of the nearest Weather Bureau station, or some other 

 convenient meteorological records. But the measurements made in the last 

 two years at our stations A, B, and C show marked and permanent daily, 

 monthly, and annual differences, although these locations are so near one 

 another that they would ordinarily have been called one. It is therefore 



