152 FLOTv'SR-GARDENING. 



is effected by the combined action of Ai?' (XL), Liffht (XL), 

 and Temperature. 



XL Air and Light. 



2'77. When an embryo plant (242) is formed within its inte- 

 guments, it is usually colorless, or nearly so ; but, as soon as it 

 begins to grow, that part which approaches the light (the stem) 

 becomes colored, while the opposite extremity (the root) 

 remains colorless. 



278. The parts exposed to the air absorb oxygen at night, 

 absorb carbonic acid and part with oxygen again in daylight ; 

 and thus in the daytime purify the air, and render it fit for the 

 respiration of man. 



279. The intensity of this latter phenomenon is in proportion 

 to the intensity of solar light to which leaves are directly ex- 

 posed. 



280. Its cause is the decomposition of carbonic acid, the 

 extrication of oxygen, and the acquisition by the plant of carbon 

 in a solid state ; from which, modified by the peculiar \dtal 

 actions of species, color and secretions are supposed to result. 



281. For it is found that the intensity of color, and the 

 quantity of secretions, are in proportion to the exposure to light 

 and air, as is shown by the deeper color of the upper sides of 

 leaves, etc. 



282. And by the fact that, if plants be grown in air from 

 which light is excluded, neither color nor secretions are formed, 

 as is exemplified in blanched vegetables ; w^hich, if even natu- 

 rally poisonous, may, from want of exposure to light, become 

 wholesome, as Celery. 



283. When any color appears in parts developed in the dark, 

 it is generally caused by the absorption of such coloring matter 

 as preexisted in the root or other body from which the blanched 

 shoot proceeds, as in some kinds of Rhubarb when forced. 



284. Or by the deposition of coloring matter formed by 



