750 PHYSIOLOGY. 



the proportion of plants to a particular area carefully considered, 

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

 cial to health by the dampness they produce ; on the other, a 

 deficiency or want of them will produce an equally injurious dry- 

 ness. 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 well- 

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

 is more abundant in those regions which are well covered with 

 forests, than in those which are comparatively free from them. 

 It is found, accordingly, that a great change may be produced in 

 tlie climate of a country by clearing it, and that while an exces- 

 sive amount of vegetation is injurious to the healthy growth of 

 plants, if the country be altogether deprived of it, it will become 

 entirely barren from extreme dryness. By inattention 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 were formerly remarkable for their fertility are now barren 

 wastes ; and, in like manner, many districts, formerly 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 cf 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. 364), Sarracenia {fig. 365), and 

 Heliamphora {fig. 366), watery excretions of this nature always 

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

 Marantacese, Musacese, Aracese, Graminaceae, &c., water is con- 

 stantly excreted in drops at certain periods of vegetation ; but 

 this may be due to the great force of absorption in certain cases. 

 The most remarkable plant of this kind, however, is the Cala- 

 dium distillatorium, from which half a pint of fluid has been 

 noticed to drop away during a single night, from orifices placed 

 at the extremities of the leaves, and communicating freely with 

 internal passages. 



2. Absorption of Fluids by Leaves. — This matter has already 

 been considered when treating of the Functions of the Epidermal 

 Tissue and its appendages (see page 742), and need not be 

 further alluded to. 



3. Absorption and Exhalation of Gases by Leaves. — We have 

 already noticed (p. 744) the property possessed by the roots of 

 absorbing liquid food from the medium in which they grow, and 

 also their supposed power of excretion (p. 745). Whilst plants 

 are thus intimately connected by tlieir roots with the soil or 

 medium in which they are placed, they have also important rela- 



