INFLUENCE OF CLIMATE. 37 



CHAPTER II. 



Influence of climate Light and heat Chemical actions- 

 Zones Isothermal lines Summer temperature Eqii- 

 noctial zone Baobab Age of trees Bombax Mahogany 

 Orchis Climbers Rafilesia Tropical forests Cacao- 

 nut Woorali poison Cassava Palms Cocoa-nut Sago 

 palm Arborescent grasses Bamboo Sugar-cane Cotton 



Scitamineae Gutta percha Camphor Spice Islands 



Nutmeg Clove Cinnamon Pepper Nettles Upas 

 Java The forbidden fruit. 



HAVING endeavoured to trace the influence 

 of soil and situation on the growth of plants, 

 we shall now consider the influence of climate 

 upon them. "We have already stated, that light 

 and heat are two of the most important agents 

 in influencing the vegetation of a country, and 

 as these are more intense in proportion as we 

 approach the equator, there vegetation is more 

 luxuriant ; and as all the conditions of their 

 growth are thus most fully met, the greater 

 number of species met with there will not sur- 

 Diise us. But it may be well to explain the 

 way in which light and heat act upon plants. 

 Plants are nourished by carbonic acid, which 

 they derive partly from the soil and partly from, 

 the air, and though the latter contains this gas 

 only in the proportion of one part in a thou- 

 sand, yet this is sufficient to answer all the 

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