224 NATURALISATION OF FORESl TREES. 



by the mean temperature during the growing season and its dura- 

 tion. Thus 50° F. is the mean temperature during the growing 

 season at the Hmits of the forest both on mountains at the equator 

 and on the borders of the Arctic regions, though the latter's mean 

 annual temperature is, I beheve, below the freezing point. Prof. 

 Mayr implies that the localities of all species of a certain genus 

 throughout the world have approximately the same mean tem- 

 perature for the growing season. For instance, the latter, in the 

 case of the Beeches, is always between 59° and 64° F. 



This may be of practical value in the case of a small genus such 

 as Fagus, but is obviously of no practical value in the case 

 of a large genus, such as Pinus, which is found in localities whoss 

 mean temperatures during the growing season probably differ in 

 as much as 15° or 20° F. What is probably of the utmost im- 

 portance is that the locality to which a certain species is 

 introduced and its native locality should have the same mean 

 temperature during the growing season within very narrow limits. 

 Possibly the mean maxima and mean minima temperatures during 

 the growing season would be still more subtle and reliable guides 

 than the mean temperature, but probably the latter is sufficient 

 for practical purposes. Not only does a species require a certain 

 minimum mean temperature, during the growing season, but 

 also it cannot endure more than a certain mean maximum tem- 

 perature during the hottest part of the year. It is probable 

 that the latter fact is the chief explanation of the failure of some 

 European species in this country, such as Pinus sylvestris. 



The mean temperatures during the dormant season are of 

 no consequence. No proof of the importance of absolute minimum 

 temperature is required in South Africa, after the numerous 

 attempts to grow sub-tropical species in the cold interior, but 

 it should be mentioned here that the minimum temperature 

 which a certain species will endure varies greatly with the warmth 

 and moisture, of the soil, the atmospheric humidity', and the 

 presence or absence of wind. 



The average difference between the mean temperatures of the 

 hottest and coldest months is a very useful guide to the extent 

 to which a climate is a continental one. On the Pacific Coast 

 of America the range is about 25° F., while on the Atlantic Coast 

 it is 45° F. in the same latitude, and it is not surprising that 

 species from the former will not grow in the latter. The range 

 of temperature by itself is, however, of no use in the selection of 

 species. It is roughly the same here as in England. '~<si 



The liability of a locality to late and early frosts, together with 

 the duration of the growing season of a species, is of much im- 

 portance. The shorter the natural growing season of an exotic 

 species is, the less is the danger from these frosts, but it must 

 be remembered that the length of its growing season, though 

 largely hereditary, will be affected by season of rainfall and 

 local changes in soil moisture. In a climate with wet summers, 

 a species is more likely to suffer from early frosts and in a climate 

 with early spring rains, it is more likely to suffer from late frosts. 



Generally speaking it is the safest to introduce a species to 



