OF THE LEAVES. 7^9 



dryness ; thus, if two plants of the same nature are submitted 

 to similar conditions, except that one is placed in a dry atmo- 

 sphere, and the other in a moist, the former will give off more 

 fluid than the latter. The great agent, however, which influ- 

 ences transpiration is light. According to De Candolle, light 

 is the only agent which is capable of promoting and modifying 

 transpiration. He says, " If we take three plants in leaf, of the 

 same species, of the same size, and of the same degree of vigour, 

 and place them, after weighing them carefully, in close vessels, — 

 one in total darkness, the other in the diSused light of day, and 

 the third in the sunshine, and prevent absorption by the roots, 

 we shall find that the plant exposed to the sun has lost a great 

 quantity of water, that in common daylight a less amount, and 

 that which was in total darkness almost nothing." The experi- 

 ments of Henslow, Daubeny, and others, also demonstrate, in a 

 most conclusive manner, the great influence of light upon tran- 

 spiration. Daubeny, moreover, found that the different rays of 

 the solar spectrum had a varying influence, the illuminating rays 

 having more effect than the heating rays. 



Light being thus shown to be the main agent concerned in 

 influencing and modifying transpiration, this process will neces- 

 sarily vary in amount, not only in the same latitudes with dif- 

 ferent degrees of light, but also in different latitudes according 

 to the intensity of the light which is found in them respectively. 

 Hence, under similar circumstances, the amount of transpiration 

 from a given surface will be greater in tropical and warm regions 

 where solar light is most intense, than in temperate and cold 

 ones ; and thus we see one reason why plants of those climates 

 are frequently protected from an excessive and injurious exliaia- 

 tion by certain special adaptations of their epidermal tissues and 

 appendages. (See p. 741.) 



The quantity of fluid thus exhaled has been the subject of 

 various experiments. The most complete observations upon this 

 point were made by Hales as long ago as 1724. He found that 

 a common Sun-flower 3^ feet high, weighing 3 pounds, and with 

 a surface estimated at 5,616 sqiiare inches, exhaled, on an 

 average, about twenty ounces of fluid in the course of the day ; a 

 Cabbage-plant, with a surface of 2,736 square inches, about 

 nineteen ounces per day; a Vine, with a surface of 1,820 square 

 inches, from five to six ounces ; and a Lemon-tree, exposing 

 a surface of 2,557 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 almost incalculable quantity must be ex- 

 haled by the whole vegetation of the globe. It can readily also 

 be understood that the air of a thickly wooded district will be 

 always in a damp condition, while that of one with scanty vege- 

 tation will be comparatively free from humidity : and hence it 

 will be seen that a country, to be perfectly healthy, should have 



