OF THE LEAVES. 735 



modifications to which it is liable, have also an important influ- 

 ence upon the transpiration of fluid matters. (See p. 728.) 



From some interesting experiments of M. Gan-eau on the 

 transpiration by leaves, he was led to draw the following con- 

 clusions : — 1. That the quantity of water exhaled by the upper 

 and under surfaces of the leaves is usually as 1 to 2, 1 to 3, or 

 even 1 to 5, or more. The quantity has no relation to the posi- 

 tion of the surfaces, for the leaves, Avhen reversed, gave the same 

 results as when in their natural position, 2. There is a corre- 

 spondence between the quantity of water exhaled and the 

 number of the stomata. 3. The transpiration of fluid takes place 

 in greater quantity on the parts of the epidermis where there is 

 least waxy or fatty matter, as along the line of the ribs. 



This transpiration of fluids is influenced to some extent by 

 the varying conditions of the atmosphere as to moisture and 

 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 CandoUe, 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 diff"used 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 ex- 

 periments of Henslow, Daubeny, and others, also demonstrate, 

 in a most conclusive manner, the great influence of light upon 

 transpiration. Daubeny, moreover, found that the different 

 rays of the solar spectrum had a varying influence, the illumi- 

 nating rays having more eff"ect 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 different 

 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 tropicid and wami 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 exhalation 

 by certain special adaptations of their epidermal tissues and 

 appendages. (See p. 728.) 



The quantity of fluid thus exhaled has been the subject of 

 various experiments. The most complete observations upon 

 3 B 



