The characteristic colour of living vegetation is green. This is also the rarest of colours among the higher animals.[21] It is due in vegetables to the presence in their cells of grains of the substance known as chlorophyll, which very few animals possess or have need of. It is developed normally under the action of sunlight, and plays a most important part in the economy of the plant. The usual method by which any organism obtains the energy necessary for its vital functions is through the oxidization, _i.e._ the slow burning, of its substance, by combination with the oxygen of the air. The process is to all intents the same as the more rapid oxidization, under great heat, of coal in a steam-engine. If a plant can obtain sugar, which oxidizes easily in contact with atmospheric oxygen, it has thus a fund of energy to draw on for all the processes of its life. Now sugar is composed of carbon and water. Carbon exists in the air, in combination with oxygen, in the form known as carbonic acid. Chlorophyll, in some way as yet unexplained, enables plants, when acted upon by light, to take in carbonic acid from the air and to disintegrate it into its constituents, carbon and oxygen. The oxygen disappears again in the air, and the released carbon combines with water in the plant to form sugar,[22] thus giving the plant its needed store of potential energy. All it does with this energy is to live, grow, and reproduce its kind; till at length a time comes when the assimilative energy weakens relatively to the forces of decay, and the plant dies; it is again resolved into the chemical constituents from which it was built up; but not without having passed on the flame of life to burn afresh in its descendants. Plants which have no chlorophyll, like certain bacteria and moulds, and which, therefore, cannot decompose the carbonic acid in the air for their nourishment, offer an interesting example of the manner in which Nature contrives to get her way, if not by the normal instruments, then by the utilization of others. They acquire their first store of energy sometimes like animals, from other organic compounds, or they take carbon from acetates and tartrates. The nitro-bacteria appear to depend on ammonia derived from decaying animal matter, and the moulds draw their energy from sugar, which (as in our jams, etc.) they find already formed.