166 AIR AND LIGHT. 



as it begins to grow, that part which approaches the light 

 (the stem) becomes, coloured, while the opposite extremity 

 (the root) remains colourless. 



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 day-time purify the air, and render it fit for 

 the respiration of man. 



279- The intensity of this latter phenomenon is in propor- 

 tion to the intensity of solar light to which leaves are directly 

 exposed. 



2S0. Its cause is the decomposition of carbonic acid, the ex- 

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

 in a solid state : from which, modified by the peculiar vital 

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



281. For it is found that the intensity of colour and the 

 quantity of secretions are in proportion to the exposure to 

 light and air, as is shown by the deeper colour. of the upper 

 side of leaves, &c 



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

 which light is excluded neither colour nor secretions are 

 formed, as is exemplified in blanched vegetables ; which, if 

 even naturally poisonous, may, from want of exposure to 

 light, become wholesome, as Celery. 



283 When any colour appears in parts developed in the 

 dark it is generally caused by the absorption of such colouring 

 matter as pre-existed 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 colouring matter formed by 

 parts developed in light, as in the subterranean rootsof Beet, 

 Carrots, &c. 



285. What is true of colour is also true of flavour, which 

 equally depends upon light for its existence ; because flavour 

 is produced by chemical alterations in the sap caused by expo- 

 sure to light. (229 ) 



286. The same thing occurs in regard to nutritive matter, 

 which in like manner is formed by the exposure of leaves to 

 light. Thus the Potato when forced in dark houses contains 

 no more amylaceous matter than previously existed in the 

 original tuber; but acquires it in abundance when placed in 

 the light, and deposits it in proportion as it is influenced by 

 light and air. Thus, also, if Peaches are grown in wooden 

 houses, at a distance from the light, they will form so little 

 nutritive matter as to be unable to support a crop of fruit, the 

 greater part of which will fall ofT. And for a similar reason it 

 is only the outside shoots of standard fruit trees that bear fruit. 

 Considerations of this kind form in part the basis of pruning 

 and training. 



287. Light is the most powerful stimulus that can be em- 

 ployed to excite the vital actions of plants, and its energy is 



