November 30, 1900.] 



SCIENCE. 



841 



the most northern in the Austro-riparian 

 area. 



The Highlands station is cited here for 

 the sake of comparison, but does not other- 

 wise answer our purpose, its elevation be- 

 ing BO great as to preclude the occurrence 



at Norfolk. In short, Norfolk temperatures 

 are farther below those of Memphis, than 

 Valley Head temperatures are below those 

 of Norfolk. The occurrence of many Aus- 

 tro-riparian species at Valley Head is there- 

 fore small matter for wonder. But in order 



of any important number of Lower Austral 

 species. Knoxville falls slightly below the 

 minimum altitude to which this discussion 

 was limited at the outset ; but owing to 

 its proximity to some of the most interest- 

 ing colonies described above, and in the 

 absence of the requisite data from points 

 lying nearer them, it has seemed best to 

 give it place in the table. The most useful 

 data are those given for Asheville and for 

 Valley Head. Both have an altitude of 

 more than 300 meters (1,000 feet) above 

 the sea, and at both points a considerable 

 number of Austro-riparian forms is known 

 to occur. 



At Asheville the normal sum total of 

 effective heat is only about 80 per cent, of 

 that at Norfolk, and slightly more than 66 

 per cent, of that at Memphis. The normal 

 number of days of the year possessing 

 physiologically effective temperatures is, at 

 Asheville, about 85 per cent, of that at 

 Norfolk, and about 82 per cent, of that at 

 Memphis. At Valley Head, which is only 

 about one- half as high as Asheville, and is 

 considerably farther south, the normal sum 

 total of heat stands to that of Norfolk in 

 about the ratio of 11 to 12 ; and, to that at 

 Memphis nearly as 4 to 5. The normal 

 number of days of the year whose temper- 

 ature exceeds 6° C. is only two less than 



to explain their presence at Asheville, and 

 at other points along the French Broad 

 River at elevations of 330 to 600 meters 

 (1,100 to 2,000 feet),* where we find the 

 temperature conditions as ordinarily ex- 

 pressed so different from those of the Aus- 

 tro-riparian area proper, other elements of 

 the milieu must be brought into considera- 

 tion. The two factors which are probably 

 most effective in permitting those species 

 to maintain themselves in what would seem 

 to be an unfriendly environment are : (1) 

 The amount of insolation ; and (2) The na- 

 ture of the soil. 



1. Insolation. — A. favorite situation in 

 the mountains for colonies of Lower Aus- 

 tral species is on the southern exposure of 

 hills, where the angle of inclination and the 

 position with reference to the sun insure 

 the greatest possible amount of insolation. 

 The duration and intensity of the heat and 

 light which such exposures receive from the 

 sun on summer days must go far towards 

 counterbalancing the effect of altitude in 

 lowering the temperature during the hours 

 of darkness, and in shortening the growing 

 season. The flora of the Coastal Plain 



*At Biltmore, N. C, with an altitude of 1,993 to 

 2,150 feet, occur Arundinaria macrospenna, A. tecta, 

 Hypericum virgatum, Helenium nudiflorum and several 

 other characteristic Lower Austral species. 



