EFFECT OF LIGHT ON PLANTS AND ANIMALS. 21 



conies, refuse to grow in the shade. Or, if one grows in 

 the shade, it has none of the sensible properties, neither 

 the strength, nor hardness, nor color, nor smell, nor taste, 

 which it would have had growing in the sunshine. 



In the growth of a tree, the stronger and fuller the 

 light to which it is exposed, the greater the amount of 

 carbon which is formed into its texture, and the harder 

 and more compact its wood. 



67. A single experiment shows that it is~ light and not 

 air which gives wood its strength and hardness. Plant 

 a little tree in a dusky room, with two openings, one 

 admitting light but no air, the other air but no light, 

 and all the little branches will soon turn towards the 

 light. 



68. This seems to be because on the side of a branch 

 towards the light, wood is formed, the growth is checked, 

 and the branch hardened ; on the other side, growth con 

 tinues more rapidly, and the parts lengthen, and thus 

 bend the little branch over towards the harder side. 



During very warm, moist nights, plants may grow in 

 length and in every other dimension. In the sunlight 

 only do they form wood. Hence it is that in seasons of 

 unusual sunshine, the wood in a tree fully exposed to the 

 sun is formed with more than common perfection, as is 

 also the bark. 



69. The power of the sun s light upon animals is not 

 less striking. The animals, beasts, birds, fishes and 

 insects, of the torrid zone, where light is intense, have 

 more activity, more vivacity, and more brilliant colors 

 than animals of the temperate and frozen zones. All 

 animals suffer from being shut up away from the light. 



70. Human Beings, not less than other animals, Suffer 

 from being kept away from Sunshine. 



