XV 



influence may be supposed most suitable for plants which 

 shed their leaves in winter, and which, like certain hyber- 

 nating animals, fall into a kind of torpor during the colder 

 months ; whereas a more equal distribution of solar heat 

 throughout the year would seem best adapted for evergreens, 

 the progress of whose growth is never altogether arrested. 



It has also been remarked, that plants whose leaves and 

 flowers remain fixed in the same direction permanently, are 

 calculated for climates where the light during summer is 

 almost continuous; whereas in the Tropics species might 

 be expected to occur, distinguished by that alternate open- 

 ing and closing of these organs which characterizes what 

 has been termed their waking and sleeping conditions, and 

 harmonizes with the alternation of day and night in the 

 climates of which they are natives. 



Hence we need not be surprised to find that, in the 

 countries with which we Europeans are most familiar, the 

 vegetable productions that occur should be limited to such 

 as are either able to sustain a considerable degree of cold, 

 or to endure a long interruption of their vital functions. 

 Nor is it difficult to explain, why succulent plants should 

 prevail in countries visited with long periods of drought. 

 They alone, either from the deficiency of stomata, or from 



