TRANSPIRATION OF LEAVES. 797 
of the leaves is usuaily as 1 to 2,1 to 3, or even 1 to 5, or more. 
The quantity has no relation to the position of the surfaces, for 
the leaves, when reversed, gave the same results as when in their 
natural position. 2. There is a correspondence 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 fluid is influenced to a great 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, though, according to M‘Nab, a plant ex- 
posed to the sun will transpire most in a moist atmosphere ; 
while in the shade, an atmosphere loaded with vapour causes 
transpiration to cease. 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 diffused 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 experiments of Henslow, Daubeny, and others, also de- 
monstrate, 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 
illuminating rays having more etfect than the heating rays. 
Transpiration has been studied by M. Weisner in three ways: — 
(1) By comparing that of green with that of bleached plants ; 
(2) by exposing plants to the solar spectrum ; (3) by placing 
them behind solutions of chlorophyll. The result of these 
experiments has been that the action of light on transpiration 
is greatly increased by the presence of chlorophyll ; that they 
are not the most luminous rays, but those which correspond to 
the absorption band of the chlorophyllian spectrum, which 
excite transpiration ; and finally, that the rays which passed 
through the chlorophyll solution exerted but little effect on 
transpiration. 
Transpiration in some cases seems to depend but little upon 
whether the stomata are open or closed, though it is generally 
greater on the under surface of leaves—i.e. where the stomata 
are chiefly found. In summer transpiration is more active than 
absorption, while in spring the reverse condition obtains. 
The quantity of fluid thus exhaled or transpired by the leaves 
