EXHALATION OR TRANSPIRATION. 105 



Special Functions of Leaves. 



203. Leaves expose the fluids of the plant to the influence of air 

 and light, and their spiral arrangement enables them to do so effectu- 

 ally. They are concerned in the elaboration of the various vegetable 

 secretions, in the formation of wood, and in the absorption of fluid and 

 gaseous matters. A plant, by being constantly deprived of its leaves, 

 will ultimately be destroyed. On this principle, weeds, with creeping 

 stems and vigorous roots, which are with difficulty eradicated, may be 

 killed. In the cells of the leaves changes take place under the agency 

 of light, by which oxygen is given off and carbon fixed. These will 

 be considered under the subject of vegetable respiration. The absorp- 

 tion of carbonic acid and of fluids is carried on by the leaves, chiefly 

 through their stomata, according to Bonnet. Some physiologists have 

 expressed doubts as to absorption being carried on by the leaves in 

 ordinary circumstances. Leaves also give off gases and fluids by a 

 process of exhalation or transpiration. Carbonic acid, to a moderate 

 extent, is exhaled during darkness, and a large quantity of fluid is 

 given off by transpiration. The number and size of the stomata regulate 

 the transpiration of fluids, and it is modified by the nature of the 

 epidermis. In plants with a thick and hard epidermal covering, ex- 

 halation is less vigorous than in those where it is thin and soft. Some 

 succulent plants of warm climates have a very thick covering. The 

 peculiar character of the leaves or phyllodia of Australian plants, is 

 probably connected with the dry nature of the climate. While heat acts 

 in promoting evaporation, the process of transpiration is more under 

 the influence of light. It assists the process of endosmose, by rendering 

 the fluid in the cells thicker, and thus promotes the circulation of sap. 



204. The quantity of fluid exhaled varies in amount in different plants. 

 A Sunflower, three feet high, gave off twenty ounces of watery fluid 

 daily. Hales found that a Cabbage, with a suface of 2,736 square 

 inches, transpired at an average nineteen ounces; a Vine, of 1,820 square 

 inches, from five to six ounces. Experiments on exhalation maybe made 

 by taking a fresh leaf with a long petiole, putting it through a hole in 

 a card which it exactly fits, and applying the card firmly and closely 

 to a glass tumbler, about two-thirds full of water, so that the petiole 

 is inserted into the water, then inverting an empty tumbler over the 

 leaf, and exposing the whole to the sun, the fluid exhaled will be seen on 

 the inside of the upper tumbler. The experiment may be varied by 

 putting the apparatus in darkness, when no exhalation takes place, or 

 in diffuse daylight, when it is less than in the sun's rays. This process 

 of exhalation imparts moisture to the atmosphere, and hence the dif- 

 ference between the air of a wooded country and that of a country 

 deprived of forests. The cells in the lower side of a leaf where stomata 



