136 AIR AND LIGHT. 



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 secre- 

 tions 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 sides 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 gen- 

 erally 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 roots of Beet, Carrots, &c. 



285. What is true of colour is also true of flavour, which equally de- 

 pends upon light for its existence ; because flavour is produced by chemi- 

 cal alterations in the sap caused by exposure to light (229). 



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

 manner is formed by 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 deposites 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 off. 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 employed to ex- 

 cite the vital actions of plants, and its energy is in proportion to its intens- 

 ity ; so that the direct rays of the sun will produce much more powerful 

 effects than the diffused light of day. 



288. Hence, if buds that are very excitable are placed in a diffused light, 

 their excitability will be checked. 



289. And if buds that are very torpid are exposed to direct light, they 

 will be stimulated into action. 



290. So that what parts of a tree shall first begin to grow in the spring 

 may be determined at the will of the cultivator. 



291. This is the key to some important practices in forcing. 



292. This should also cause attention to be paid to shading buds from 

 the direct rays of the sun in particular cases ; as in that of cuttings, whose 

 buds, if too rapidly excited, might exhaust their only reservoir of sap, the 

 stem, before new roots were formed to repair such loss. 



293. As plants derive an essential part of their food from the air (280) 

 by the action of light, it follows that in glass houses those which admit 

 the greatest portion of light are the best adapted for purposes of cultiva 

 tion. 



294. The proportion of opaque matter in the roof of a glass house con- 

 structed of wood varies from one third to one seventh ; that of an iron 

 house does not exceed one twenty-third. 



