PERSPIRATION. 137 



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 sulphurous acid gas is quickly fat;d 

 to the life of plants ; and hence the danger of flues heated by coal fires, 

 and the impossibility 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 constantly 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 su^h copious perspiration is the separation 

 and solidification of the carbonized matter that is produced for the peculiar 

 secretions of a species. 



302. For the maintenance of a plant in health, it is indispensable that 

 the supply of fluid by the roots should be continual and uninterrupted. 



303. If any thing causes perspiration to take place faster than it can be 

 counteracted by the absorption of fluid from the earth, plants will be dried 

 up and perish. 



304. Such causes are, destruction of spongioles, an insufficient quantity 

 of fluid in the soil, an exposure of the spongioles to occasional dryness, 

 and a dry atmosphere. 



305. The most ready means of counteracting the evil consequences of 

 an imperfect action of the roots is by preventing or diminishing evapora- 

 tion. 



306. This is to be effected by rendering the atmosphere extremely humid. 



307. Thus, in curvilinear iron hot-houses, in which the atmosphere be- 

 comes so dry in consequence of the heat, that plants perish, it is necessary 

 that the air should be rendered extremely humid, by throwing water upon 

 the pavement, or by introducing steam. 



308. And in transplantation in dry weather, evergreens, or plants in leaf, 

 often die, because the spongioles are destroyed, or so far injured in the 

 operation as to be unable to act, while the leaves never cease to perspire. 



309. The greater certainty of transplanting plants that have been grow- 

 ing in pots, is from this latter circumstance intelligible. 



310. While the utility of putting cuttings or newly transplanted seed- 

 lings into a shady damp atmosphere, is explained by the necessity of hin- 

 dering evaporation. 



