Plants are those organized beings that live directly upon the mineral kingdom, upon the surrounding earth, air, water. They alone convert inorganic, or mineral, into organic matter; whilst animals originate none, but draw their whole sustenance from the organized matter which plants have thus elaborated. Plants, having thus the most intimate relations with the mineral world, are generally fixed to the earth, or other substance upon which they grow, and the mineral matter upon which they feed is taken directly into their system by absorption from without, and is assimilated under the influence of light in organs exposed to the air, while animals, endowed with volition and capable of responding promptly to external impressions, have the power of selecting the food ready prepared for their nourishment, which is received into an internal reservoir or stomach. The permanent fabric of plants is composed of only Carbon, Hydrogen, and Oxygen. The tissue of animals contains an additional element, viz., Nitrogen. Plants, as a necessary result of assimilating their inorganic food, decompose carbonic acid and restore its oxygen to the atmosphere. Animals in respiration continually recompose carbonic acid, at the expense of the oxygen of the atmosphere and the carbon of plants.
CHAPTER XVII.
ORGANIC LIFE—ANIMAL.
We have seen that the principal elements, the most active, that enter into the composition of plant life, that form the food substance for the support and nourishment of animals, are mainly composed of three elements, Oxygen, Hydrogen, and Carbon; that during evolution, growth, and development certain elements are absorbed and assimilated, while others, the gases, are exchanged. Plants yield up Oxygen and take in Carbonic acid from the atmosphere, which they store up and elaborate.
We have also seen that all the elements that enter into the composition of the various sorts of vegetation, are, chemically considered, seventeen in number.
ANIMAL LIFE.
The animal, like the vegetable, is also composed of chemical elements, and by chemical analysis has been found to contain eighteen, as follows:
1. Of primary or vital importance: Oxygen, Hydrogen, Carbon, Nitrogen.