736 PHYSIOLOGY. 



this point were made by Hales. He found that a common Sun- 

 flower 3^ feet high, weighing 3 pounds, and with a surface esti- 

 mated at 56 1 6 square inches, exhaled on an average, about twenty 

 ounces of fluid in the course of the day ; a Cabbage-plant, with 

 a surface of 2736 square inches, exhaled about nineteen ounces 

 per day ; a Vine, with a surface of 1820 square inches, from five 

 to six ounces ; and a Lemon-tree, exposing a surface of 2557 

 square inches, six ounces on an average in a day. If such a 

 large amount of fluid be thus given off" by single plants, what 

 an enormous and almost incalculable quantity must be exhaled 

 by the whole vegetation of the globe, and it is in this manner 

 that a large supply of moisture is constantly thrown off into the 

 atmosphere ; and hence we can readily understand one cause 

 which leads to the difference between the air of a thickly wooded 

 country and that in which vegetation is more or less scanty, 

 for the former will be always in a damp condition, while the 

 latter will be comparatively free from humidity. Thus, we see 

 that a country to be perfectly healthy, should have the propor- 

 tion of plants to a particular area carefully considered, for 

 while, on the one hand, too many plants are generally preju- 

 dicial to health by the dampness they produce ; on the other, 

 a deficiency or want of them, wiU produce an equally injurious 

 dryness. The same circumstances have an important bearing 

 upon the fertility or otherwise, of the soil, and thus have an 

 indirect influence upon the health of the inhabitants. Thus, it 

 is a fact, that as vapour is constantly given off by plants, rain is 

 more abundant in those regions which are freely supplied with 

 forests, than in those Avliich are comparatively free from them, 

 and we find accordingly, that a great change may be produced 

 in the climate of a country by clearing it of forests, for while 

 an excessive amount of vegetation is injurious to the healthy 

 growth of plants, if the country be deprived of them, it will 

 become entirely barren from its extreme dryness. By inatten- 

 tion to these simple but most important facts, which clearly 

 indicate, that open land and that furnished with plants should 

 be properly proportioned the one to the other, many regions of 

 the globe which Avere formerly remarkable for their fertility, 

 are now barren wastes ; and in like manner, many districts for- 

 merly noted for their salubrity, have become almost, or quite, 

 uninhabitable. 



The fluid which thus passes off by the leaves of plants is 

 almost pure water. This transpiration of watery vapour must 

 not be confounded with the excretion of water containing 

 various saline and organic matters dissolved in it, which takes 

 place in certain plants, either from the general surface of the 

 leaves or from special glands. In the peculiar formed leaves of 

 Dischidia, Nepenthes {fig. 367), Sarracenia {fig. 368), and 

 Heliamphora {fig. 369), watery excretions of this nature always 

 exist. From the extremities or margins of the leaves of various 



