158 THE. GEOGRAPHICAL DISTRIBUTION 



whose snowy peaks are seen, even in tropical countries," 

 but also to those plateaus or elevated table-lands, which 

 abound in different parts of the world. The water with 

 which a vast portion of the earth's surface is overspread 

 greatly modifies its temperature. Countries situated in 

 the neighborhood of the ocean, are always cooler than 

 those which are removed from its influence. The iso- 

 thermal lines are not therefore parallel to the equator, 

 excepting in the neighborhood of the equinoctial line, but 

 form an irregular curve around the earth's surface. Tem- 

 perature undoubtedly influences the geographical distribu- 

 tion of plants. To every species of plant there is a certain 

 degree of temperature necessary before it will germinate. 

 Hence it is that every month produces its own flora, the 

 result of the ever-varying temperature of the year, anij 

 each zone produces its owa vegetation which would not 

 flourish elsewhere. 



Light. — The influence of light on vegetation is perhaps 

 not so great as that of temperature, yet it is nevertheless 

 deserving of an especial notice. The decomposition and 

 consolidation of the elementary food of plants, the forma- 

 tion of the green parts, the exhalation of moisture by 

 their leaves, its absorption by their roots, and all the other 

 circumstances of vegetable life, are owing to the illumi- 

 nating power of the sun. In tropical countries, where the 

 light of the sun is the most powerful, we meet with plants 

 which have the most intense colors, the strongest odors, 

 and the most active properties. These plants when culti- 

 vated in the stove, never acquire the fragrance and virtues 

 which they possessed in their native country ; for although 

 we can place them in an atmosphere of the same tempera- 

 ture as their own in these northern climates by applying 



