3 TO 
the light. Some plants, as the curious tribe of cacti, have no leaves; the 
green stems of leafless plants perform the functions of leaves. 
When the crude sap reaches leaves exposed to the action of light, wa¬ 
ter is thrown off. The quantity lost by various plants was determined 
by Hales, one of the earliest inquirers into the physiology of plants, and 
lie estimated the average rate of perspiration of plants to be equal to 
17 times that of a man. The amount of perspiration depends mainly on 
the intensity of the light, most water being thrown off during bright 
sunshine, and none in total darkness. It is in the leaves that water and 
the carbonic acid absorbed by the roots from the soil and by the leaves 
from the air are decomposed, the hydrogen of water, and the carbon of 
the carbonic acid being retained or assimilated by the plant, and the oxy¬ 
gen which was combined with the carbon and hydrogen is set free. The 
decomposition of water and carbonic acid depends also on the action of 
light on leaves. Priestly, towards the close of the last century, discovered 
that if plants were placed in air in which an animal, as a mouse, had died, 
and the plant was exposed to light, that the air in the vessel was again 
rendered fit for breathing. By the breathing of animals oxygen is taken 
from the air and carbonic acid added to it. If an animal is placed in an 
atmosphere of carbonic acid it instantly dies, so if it is confined to a 
limited quantity of atmospheric air in an air-tight vessel the oxygen of 
the air is speedily consumed, a quantity of carbonic acid produced, and 
the death of the animal is the consequence. Carbonic acid is produced 
foy the decay of animal and. vegetable bodies, by combustion, fermenta¬ 
tion, &c., as well as by the respiration of animals; yet this gas consti¬ 
tutes only about the one-thousandth part of the atmosphere. It has 
been wisely ordered that plants should take from the air carbonic acid 
which in excess is injurious to animal life, and that they should give back 
the oxygen which is essential to animal existence. 
It has been proved by many and variously combined experiments, that 
the leaves of plants when exposed to light have the power of decompos¬ 
ing carbonic acid. One of the neatest and most conclusive is that of the 
distinguished botanist, De Candolle. He placed two inverted glass re¬ 
ceivers in a cistern, filled with distilled water, the water being protected 
from the air by a layer of oil, which floated about its surface. One glass 
was filled with distilled water and had a plant of water mint floating in it, 
the other was filled with carbonic acid. When the apparatus was exposed 
