Xll 



To account, for instance, for the occurrence of arbores- 

 cent Endogens in the Tropics, of herbaceous ones in the 

 Temperate Zones of the earth; for the predominance of 

 evergreen trees in the former, and of deciduous ones in the 

 latter ; for the greater profusion of annuals and perennials 

 where the transition is great between the heat of summer 

 and the cold of winter; for the prevalence of succulent 

 plants in certain regions, as of the Cactea in Tropical Ame- 

 rica, or of certain Euphorbiacea in Africa, are problems 

 perhaps of not very difficult solution. 



If we take the Palm tribe for example, we may appeal to 

 the juicy and spongy character of the internal portions of 

 their stem, and to the want of that protection against frost 

 afforded in trees of exogenous growth by their numerous 

 layers of bark, with intervening spaces occupied by air, 

 which interpose a non-conducting substance between the 

 external atmosphere and their sap, as accounting for the 

 greater susceptibility of these plants to the influence of cold, 

 and for the greater chance of damage being incurred by 

 their delicate tissues, arising from the rending of their cells 

 through the expansion caused by frost. 



We can also understand why trees and shrubs which 

 strike their roots deep into the ground should resist the 



