1892.] 1' [Heilprin. 



perpetual snow is the determinant of absolute or greatest elevation, but 

 this is not strictly the case. Thus, it is well known that in the Swiss 

 Alps phanerogamic plants are found nearly 2700 feet above the snow line ; 

 the beautiful mountain pink (Silene acauUs) has been met with at an 

 elevation of 11,382 feet,* and Androsace glacialis, a primulaccous plant, 

 at 11,406 feet, on the Piz Linard (Grisons). Indeed, Heer has determined 

 not less than a hundred species (or approximately that number) of flow- 

 ering plants (representing twenty-three families) as growing on the 

 Rhaetic Alps above the snow line (9060 feet), and Martins has recorded 

 twenty-four species from the Grands Mulcts, Mont Blanc, on elevations 

 ranging from 10,540 to 11,300 feet.f 



So lar as the Mexican summits are concerned, I think it may be safely 

 asserted that the tree or timber line is not an absolute one ; in other words 

 it is not one which is determined by the natural conditions of growth of 

 the plant itself, but rather it is dependent upon purely local causes. It 

 is scarcely conceivable, for example, that on Orazaba, where at an eleva- 

 tion of upwards of 13,200 feet the trees were still 30-40 feet in height, 

 an additional 500-600 feet should so materially alter favorable (climatic) 

 conditions of growth into unfavorable ones as to produce extermination ; 

 indeed, we must assume that this change is even much more rapid, for at 

 the very verge of the timber line the pines, although necessarily harboring 

 a considerable number of small specimens, still easily measured 20-30 

 feet. This condition we found repeated on Popocatepetl and Ixtaccihuatl, 

 most markedly, perhaps, on the latter mountain ; I am positive that some 

 of the uppermost pines here, very close to the disappearing line, were not 

 less than 40-50 feet high, if not higber. Again, on Popocatepetl, as has 

 already been remarked, the timber line ceases a little above 13,100 feet, 

 the trees themselves being of rather inconsiderable height. On an equiv- 

 alent height on a spur of the Sierra Tlamacas,J however, the pines are 

 still nolile foresters, and on the Sierra Tlamacas itself, off in the direction 

 of Ixtaccihuatl, they rise to elevations several hundred feet higher. 

 There is little doubt in my mind that the actual limitation on the summits 

 here referred to is mainly determined by such physiographic conditions 

 as steepness of slope, downwash of soil, exposure to the cold waters of 

 melting snows, storms, etc. How much higher, under more favorable 

 conditions, the tree line might have attained, I am unable to say ; but it 

 is interesting to note that such as it is, it is virtually the most elevated tree 

 line in the world. § 



* Humboldt, " Views of Nature," p. 31S. I met with this plant (summer of 1891) in 

 various parts of Greenland, between lat. 09° and 77° 40', growing from the sea-level to 

 an elevation of 1500-2000 feet. 



tGrisebach, op. cit., i, p. 167. 



I Cros.sed just before reaching the ranch of Tlamacas. 



§ This statement, perhaps, requires modification. Poppig, from manuscript data sub- 

 mitted to him by Engineer Benjamin Scott, asserts ("Keise in Chile, Peru and auf dem 

 Amazonenstrome," ii, p. 80) that on the Peruvian Andes, near the hamlets of Huaylillas 

 de Potosiand Uchusuma, treelels of (?) Polylepis racemosa are found at elevations of 15,8S3 



PUOC. AMER. PHILOS. SOC. XXX. 137. C. PRINTED MARCH 1, 1892. 



