SECT, xxvir.] LIGHT EEQUISITE FOE PLANTS. 301 



withstanding 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 na- 

 ture. In salt plains and sandy deserts alone, hopeless barren- 

 ness prevails. Plants grow on the borders of hot springs 

 they form the oases 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 everlasting ice. Those in- 

 teresting mariners narrate, that under 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, admits the sun's rays, which warm and cherish the plants 

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

 such protection unnecessary. 



The chemical action of light is, however, absolutely requi- 

 site for the growth of plants which derive their principal 

 nourishment from the atmosphere. They consume carbonic 

 acid gas, vapour, nitrogen, and the ammonia it contains ; but 

 it is the chemical agency of light that enables them to absorb, 

 decompose, and consolidate these substances into wood, leaves, 

 flowers, and fruit. The atmosphere would soon be deprived 

 of these elements of vegetable life, were they not perpetually 

 supplied by the animal creation ; while in return, plants 

 decompose the moisture they imbibe, and, having assimilated 

 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 combined influences of the 

 whole solar beams that give such brilliancy to the tropical 



