28 
the leaves consists in the interchange of gases. The air contains a 
quantity of carbonic acid gas (a compound of carbon wifh oxygen), 
and upon this, with ammonia (consisting of nitrogen and hydrogen) 
and water (oxygen and hydrogen), the plant feeds. In day- 
light, when the leaves are exposed to the sun, the3'^ are engaged 
in imbibing the carbonic acid gas from the air, and in utilising 
it. They store up the carbon, which is needed for their tissues 
and secretions, and they set free the oxygen into the air. In 
this manner plants, Avhile engaged in feeding by means of their 
leaves, act in an opposite way to animals. The latter, when breathing, 
avail themselves of the oxygen of the air, and give out from their 
lungs carbonic acid and other gases. Thus, what is of no service to 
the one, is essential to the other, and vice versa. Plants, as they feed, 
fit the air for the respiration of animals ; animals, as they breathe, 
yield up to the atmosphere the ingredients needed for the food of 
plants. The following is a simple mode of proving the emission of 
oxygen gas from plants : — Take a few leaves, place them in a tumbler 
half filled with water, invert over the mouth of the tumbler a funnel 
of glass, closing its aperture by a small cork, and place the apparatus 
in a window exposed to the sun. Shortly bubbles of gas will be 
observed on the leaves ; the gas so formed will accumulate, and 
ultimately fill the funnel. If now the cork be removed, and a lighted 
match be applied to the end of the tube, the match will burn with 
increased brilliancy, or if its flame be previously put out, and the 
wood be in a glowing state, it will again burst into flame when placed 
near the aperture of the funnel, thus proving the existence of oxygen 
gas. At night, or if exposed to darkness, the leaves cease to feed ; 
but as they continue to breathe they set free carbonic acid, and, to 
some extent, therefore, render the air impure for animal respiration. 
“ As a*result of the interchange of gases, of which we have been 
speaking, growth,, the formation of new tissues, and the production of 
various secretions, A'c., take place. It is the green colouring matter 
of the leaf, the chloroph/U, which is the chief agent in the breaking 
up of carbonic acid, the setting free of oxygen, and the fixing of the 
carbon under the influence of sunlight. It is supposed by chemists 
that the gradual reduction in tlie quantity of oxygen may account for 
the formation of starch, various vegetable acids, and other secretions, 
containing relatively less and less oxygen, till ultimately such sub- 
stances are formed as turpentine, resins, or other so-called hydro- 
carbons, wliich consist of hydrogen and carbon iu admixture, and 
into the composition of which little or no oxygen enters. 
“ The remaining phenomena of plant life may perhaps be best 
illustrated by continuing our comparison of them with those mani- 
fested in the Animal Kingdom. Anatomically, there is no line to be 
drawn between plants and animals ; tlie lowest plants and the lowest 
animals are so much alike that it often happens that the naturalist is 
unable to say to which group a particular organism may belong. 
Physiologically, how^ever, there are differences, as w'e shall now 
attempt to show. Regarded as living beings, both plants and animals 
feel, feed, digest, breathe, grow, move, and increase iu numbers. It is 
in their mode of fulfilling these functions that the chief differences 
between the two kingdoms consist. 
