264 DISTRIBUTION OF PLANTS. SECT. XXVII. 



that enables them to absorb, decompose, and consolidate 

 these substances into wood, leaves, flowers, and fruit. 

 The atmosphere would soon be deprived of these ele- 

 ments of vegetable life, were they not perpetually sup- 

 plied by the animal creation ; while in return, plants 

 decompose the moisture they imbibe, and having assim- 

 ilated the carbonic acid gas, they exhale oxygen for the 

 maintenance of the animated creation, and thus preserve 

 a just equilibrium. Hence it is the powerful and com- 

 bined influences of the whole solar beams that give such 

 brilliancy to the tropical forests, while with their de- 

 creasing energy in the higher latitudes, vegetation be- 

 comes less and less vigorous. 



By far the greater part of the hundred and ten thou- 

 sand known species of plants are indigenous in Equinoctial 

 America. Europe contains about half the number ; Asia 

 with its islands, somewhat less than Europe; New 

 Holland with the islands in the Pacific, still less ; and in 

 Africa there are fewer vegetable productions than in 

 any part of the globe of equal extent. Very few social 

 plants, such as grasses and heaths, that cover large 

 tracts of land, are to be found between the tropics, ex- 

 cept on the sea-coasts and elevated plains : some excep- 

 tions to this, however, are to be met with in the jungles 

 of the Deccan, Khandish, &c. In the equatorial regions, 

 where the heat is always great, the distribution of plants 

 depends upon the mean annual temperature ; whereas 

 in temperate zones the distribution is regulated in some 

 dogree by the summer heat. Some plants require a 

 gentle warmth of long continuance, others flourish most 

 where the extremes of heat and cold are greater. The 

 range of wheat is very great : it may be cultivated as far 

 north as the 60th degree of latitude, but in the ton-id 

 zone it will seldom form an ear below an elevation of 

 4500 feet above the level of the sea, from exuberance of 

 vegetation ; nor will it ripen above the height of 10,800 

 feet, though much depends upon local circumstances. 

 Colonel Sykes states that in the Deccan wheat thrives 

 1800 feet above the level of the sea. The best wines 

 are produced between the 30th and 45th degrees of 

 north latitude. With regard to the vegetable kingdom, 

 elevation is equivalent to latitude, as far as temperature 



