The continuation of life depends on certain chemical changes which are accompanied by a loss of substance on the part of the active tissues. This loss must be made good by the assimilation of material from without, and the manner in which assimilation takes place constitutes one great point of difference between Plants and Animals. In the majority of the former (certain fungi form the main exceptions) the cells in those portions of the plant which are exposed to the light and air contain a peculiar green substance called chlorophyll, and through the agency of this substance they are able to obtain from the inorganic kingdom nearly all the food they require. Water, with such salts as may happen to be in solution, is taken up by the roots, and carried through the stem to the leaves; here the active chlorophyll-bearing cells, under the influence of the sun's rays, cause its elements to unite with the carbon dioxide present in the air, and form various substances, of which we may take starch or cellulose as an example. The reaction may be represented chemically, thus: -

6CO2 + 5H2O = C6H10O5 + O12.

Starch or Cellulose.

A large proportion of oxygen is thus set free and discharged into the atmosphere.

The most striking property of plant protoplasm is the power of using the energy of the sun's rays to separate the elements of the very stable compounds, carbon dioxide and water, and from the elements thus obtained to make a series of more complex and unstable compounds, which readily unite with more oxygen, and change back to carbonic anhydride and water.

The carbon compounds made in and by the protoplasm of the green plants are some of the so-called "organic compounds," which enter into the composition of both plants and animals, and form an essential part of the food of the latter. They may be divided into three groups - .

1. Carbohydrates - bodies so called from the presence of hydrogen and oxygen in proportion to form water; e.g.: - .

Starch, C,H1065 = C6(H20)5.

Grape sugar (dextrose), C6H1206 = C6(H20)6. Cane sugar (sucrose), C12H22O11 = (C12H20)11.

2. Fats - compounds of carbon and hydrogen with a less proportion of oxygen than the starches, e. g.: -

Olein (principal constituent of olive oil), C57H104O6.

3. Albuminous bodies which contain nitrogen in addition to carbon, hydrogen and oxygen. These are of complex composition, and, as a rule, cannot be represented by chemical formulae.

Animals cannot thrive on the simple forms of food obtainable from the inorganic kingdom, which suffice for the nutrition of a plant. They require for assimilation materials nearly allied in chemical composition to their own tissues; substances to be used as fuel in producing the activities of their bodies. In short, they require as food the very organic substances which plants spend their lives in making: viz., starches, fats and albuminous bodies. These substances must be supplied to animals ready made, so that directly or indirectly, through the medium of other animals, all these complex substances are the result of work done by vegetable life.

The chief acts of animal protoplasm are oxidations, a slow burning away of its substance, which results in the production of inorganic materials like those used by plants as food.

Plants use simple food stuffs, and from them manufacture complex combustible materials, and thus store up the energy of the sun's rays in their textures.

Animals, on the other hand, use complex food stuffs to renew their tissues, which are constantly being oxidized, and by this means the energy for the performance of their active functions is set free.

Although the various kinds of food stuffs used by animals are highly organized and like the animal tissues in composition, yet they cannot be admitted at once into the economy without having undergone a special preparation, which takes place in the digestive tract, where the various food stuffs are so changed as to allow them to pass into the fluids of the body.

We shall first consider the food stuffs, next their preparation for absorption (digestion), and then the means by which they are distributed to the tissues (circulation). The final step in tracing the assimilation of the food is to follow the intimate processes which go on between the blood, which carries the nutriment, and the different tissues.