20 TERRESTRIAL ADAPTATIONS. 



plant. But there are various circumstances which 

 show that this recurrence of the same events and at 

 equal intervals is not entirely owing to external 

 causes, and that it depends also upon something 

 in the internal structure of vegetables. Alpine 

 plants do not wait for the stimulus of the sun's 

 heat, but exert such a struggle to blossom, that 

 their flowers are seen among the yet unmelted 

 snow. And this is still more remarkable in the 

 naturalization of plants from one hemisphere to 

 the other. When we transplant our fruit trees 

 to the temperate regions south of the equator, 

 they continue for some years to flourish at the 

 period which corresponds to our spring. The 

 reverse of this obtains, with certain trees of the 

 southern hemisphere. Plants from the Cape of 

 Good Hope, and from Australia, countries whose 

 summer is simultaneous with our winter, exhibit 

 their flowers in the coldest part of the year, as 

 the heaths. 



This view of the subject agrees with that main- 

 tained by the best Botanical writers. Thus De- 

 candolle observes that after making allowance 

 for all meteorological causes, which determine 

 the epoch of flowering, we must reckon as an- 

 other cause the peculiar nature of each species. 

 The flowering once determined, appears to be 

 subject to a law of periodicity and habit.* 



* Decandolle. Physiologic, vol. ii. 478. 



