154 FLOWER-GARDENING. 



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 one-third to one-seventh j 

 that of an iron house does not exceed one twenty-third. 



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 perpendicular sides 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 towns. 



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 twenty-four 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. 



301. The consequence of such copious perspiration is the 

 separation and solidification of the carbonized matter that is 

 produced for the peculiar secretions of a species. 



