SECT. XXVII. LIGHT REQUISITE FOR PLANTS. 263 



by the regions of the olive and vine ; these again yield 

 to the verdant meadows of more temperate climes ; then 

 follow the birch and the pine, which probably owe their 

 existence in very high latitudes more to the warmth of 

 the soil than to that of the air. But even these enduring 

 plants become dwarfish stunted shrubs, till a verdant 

 carpet of mosses and lichens, enameled with flowers, 

 exhibits the last sign of vegetable life during the short 

 but fervent summers at the polar regions. Such is the 

 effect of cold and diminished light on the vegetable king- 

 dom, that the number of species growing under the 

 line, and in the northern latitudes of 45 and 68, are in 

 the proportion of the numbers 12, 4, and 1. Notwith- 

 standing the remarkable difference between a tropical 

 and polar Flora, light and moisture seem to be almost the 

 only requisites for vegetation, since neither heat, cold, 

 nor even comparative darkness, absolutely destroy the 

 fertility of nature. In salt plains and sandy deserts 

 alone, hopeless barrenness prevails. JPlants grow on the 

 borders of hot springs they form the oasis wherever 

 moisture exists, among the burning sands of Africa 

 they are found in caverns almost void of light, though 

 generally blanched and feeble. The ocean teems with 

 vegetation. The snow itself not only produces a red 

 alga, discovered by Saussure in the frozen declivities of 

 the Alps, found in abundance by the author crossing 

 the Col de Bonhomme from Savoy to Piedmont, and by 

 the polar navigators in the Arctic regions, but it affords 

 shelter to the productions of those inhospitable climes 

 against the piercing winds that sweep over fields of ever- 

 lasting ice. Those interesting mariners narrate, that 

 ander this cold defence plants spring up, dissolve the 

 snow a few inches round, and the part above being 

 again quickly frozen into a transparent sheet of ice, ad- 

 mits the sun's rays, which warm and cherish the plants 

 in this natural hot-house, till the returning summer ren- 

 ders such protection unnecessary. 



The chemical action of light is, however, absolutely 

 requisite for the growth of plants which derive their 

 principal nourishment from the atmosphere. They con- 

 sume carbonic acid gas, vapor, nitrogen, and the ammo- 

 nia it contains ; but it is the chemical agency of light 



