106 BIOLOGY OP PLANTS. 



2. But the most direct agency of light is seen in the pow- 

 er which it gives to the leaves, to decompose the carbonic 

 acid, and prepare the juice for the processes of assimilation. 

 Plants which grow in the dark do not possess this power, and 

 hence the acid remains in the juices, and the carbon is but 

 feebly assimilated. We can see the reason why vegetation 

 is more flourishing in the torrid zone ; there is a greater 

 quantity of light there. But each plant seems to have a con- 

 stitution peculiar to itself in this respect ; some being able to 

 flourish better in the shade than others. There are some kinds 

 of grasses, and many flowers which will flourish best in the 

 shade, while others require to be exposed to all the light there 

 is, and if the natural quantity is not supplied, will wilt and 

 die. 



3. The different effects produced by rays of different colors 

 has been pointed out by Mr. Hunt. In his experiments, 

 cress seeds were exposed in the soil to the action of the 

 red, yellow, green and blue rays, which were transmitted 

 through equal thicknesses of colored infusions. " After ten 

 days, there was, under the blue fluid, a crop of cress of as bright 

 a green as any which grew in full light, and far more abun- 

 dant. The crop was scanty under the green fluid, and of a 

 pale, yellow, unhealthy color. Under the yellow solution, 

 only two or three plants appeared, but less pale than those 

 under the green ; while beneath the red, a few more plants 

 came up than under the yellow, though they also were of an 

 unhealthy color. The red and blue bottles being now mu- 

 tually transferred, the crop formerly beneath the blue in a 

 few days appeared blighted, while on the patch previously ex- 

 posed to the red, some additional plants sprang up." 



The necessity of light to the health and vigor of plants, is 

 a matter of daily observation. They seem, as it were, to be 

 endowed with a kind of instinct for it. How constantly do 

 many leaves follow the sun in his daily course ! What count- 

 .less numbers of blossoms close and drop when he retires at 



