The _green_ colour of plants is likewise found to depend on the sun’s light being allowed to shine on them; for without the influence of the solar light, they are always of a _white_ colour. It is found by experiment that, if a plant which has been reared in darkness be exposed to the light of day, in two or three days it will acquire a green colour perceptibly similar to that of plants which have grown in open day-light. If we expose to the light one part of the plant, whether leaf or branch, this part alone will become green. If we cover any part of a leaf with an opake substance, this place will remain white, while the rest becomes green. The whiteness of the inner leaves of cabbages is a partial effect of the same cause, and many other examples of the same kind might easily be produced. M. Decandolle, who seems to have paid particular attention to this subject, has the following remarks: ‘It is certain, that between the white state of plants vegetating in darkness, and complete verdure, every possible intermediate degree exists, determined by the intensity of the light. Of this any one may easily satisfy himself by attending to the colour of a plant exposed to the full day-light; it exhibits in succession all the degrees of verdure. I had already seen the same phenomenon, in a particular manner, by exposing plants reared in darkness to the light of lamps. In these experiments, I not only saw the colour come on gradually, according to the continuance of the exposure to light; but I satisfied myself, that a certain intensity of permanent light never gives to a plant more than a certain degree of colour. The same fact readily shows itself in nature, when we examine the plants that grow under shelter or in forests, or when we examine in succession the state of the leaves that form the heads of cabbages.’[6]