AIR AND LIGHT. 167 



in proportion to its intensity; 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 ex- 

 haust 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 cultivation. 



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

 house constructed of wood varies from ^ to 7,— that of an iron 

 house does not exceed - 2 l 3 -. 



295. Therefore iron roofed houses are in this respect better 

 suited for cultivation than wooden-roofed houses. 



296. And it has been found by experiment, that light passes 

 more freely through a curvilinear than through a plane roof, 

 and through glass forming an acute angle with the horizon 

 than through perpendicular glass, it follows that a curvilinear 

 roof is best, and a plane roof with glass perpendicularsides the 

 worst adapted to the purposes of the cultivator. 



297. For the same reason common green glass is less fitted 

 for glazing forcing-houses than white crown glass. 



298. Poisonous gases in very minute quantities act upon 

 vegetation with great energy. A ten-thousandth part of sul- 

 phurous acid gas is quickly fatal to the life of plants ; and 

 hence the danger of flues heated by coal fires, and the impos. 

 sibility of making many species grow in the vicinity of houses 

 heated by coal fires, or in large tow r ns. 



XII. Perspiration. 



299. It is not, however, exclusively by the action of light 

 and air that the nature of sap is altered. Evaporation is con^ 

 stantly going on during the growth of a plant, and sometimes 

 is so copious, that an individual will perspire its own weight 

 of water in the course of 24 hours. 



300. The loss thus occasioned by the leaves is supplied by 

 crude fluid, absorbed by the roots, and conveyed up the stem 

 with great rapidity. • 



