22 SEEDING AND PLANTING 



with more or less fixed limits as to its extension in either direc- 

 tion. 



Species exhibit great variation in their ability to withstand 

 cold. Thus, many tropical species die at several degrees above 

 the freezing point, while many arctic species can safely with- 

 stand 76 F. 1 The power of resisting heat also varies with the 

 species. Thus, our arctic species disappear as we proceed south- 

 ward, and our alpine species as we descend to lower elevations. 

 However, there is no part of the earth's surface so hot that no 

 species of tree will grow if other conditions are favorable. 



As regards temperature requirements, it is known that a low 

 temperature during a long growing season is not equivalent to a 

 higher temperature for a shorter duration. The total amount of 

 heat, therefore, during the growing season is not a safe guide to 

 the heat requirements of a species. In general, however, if we 

 know the temperature for the four hottest months of the year in 

 any locality, we are able to form a judgment as to what species 

 from other regions it is possible to introduce in forest practice, 

 provided we have accurate knowledge of the heat requirements of 

 the introduced species. 



8. Variation in Heat Requirements within the Species. 

 There is not only a variation between species in their power to 

 resist excessive heat or cold, but there is also a difference in the 

 individuals of the same species. 2 Thus, stock grown from hickory 

 and walnut seed collected in southern Missouri or farther south 

 will not survive the climate of Minnesota and New York, while 

 that from the same species collected farther north is perfectly 

 hardy. Stock grown from the seed of Douglas fir collected in 

 California is not hardy in New England, while that from the 

 same species in Colorado is much hardier. Black oak grown 

 from northern seed is perfectly hardy in Connecticut, but seed of 

 this species collected in Oklahoma, when planted at New Haven, 

 produces plants which will not survive the first winter. 



It is probable that in these cases natural selection working 



1 Schimper, A. F. W.: Plant-geography upon a physiological basis, p. 38. 

 Oxford, 1903. [Translated from the German by W. R. Fisher, and revised 

 by Groom and Balfour.] 



2 Engler, Arnold: Einfluss der Provenienz des Samens auf die Eigen- 

 schaften der forstlichen Holzgewachse. (Mitteilungen der schweizerischen 

 Centralanstalt f. d. forstliche Versuchswesen, VIII. Bd., S. 81-236, 1905;. 

 u. X. Bd., S. 189-214, 1913.) 



