This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Most plants with which the gardener has to deal contain chlorophyll or leaf green in their leaves and the superficial tissues of their stems, at least during the first year. It consists of a green pigment colouring the large granules that develop it and lie embedded in the protoplasm, but are capable of shifting their position if the light is too strong for them. These green granules, under the influence of sunlight and electric light, are the agents by which all raw food materials are chemically changed in character and converted into organized material suitable for building up the plant body and enabling it to store food for future use, or to provide for its offspring. The light must be accompanied by the other necessaries of plant life, such as heat, air, and moisture. Green plants are thus able to manufacture their own food and lead an independent existence. Crotons, Dracaenas, Coleus, and other plants with highly coloured leaves have chlorophyll in their tissues, but this is obscured by the presence of other colouring matters, diffused through the cell sap.