Many experiments have been tried to solve this question, but it is still a matter of doubt. If plants are placed in coloured glasses, less oxygen is disengaged than under the influence of white light. Young plants grown in comparative darkness, and consequently pale as to colour, have been exposed to different rays of the spectrum, the effect being that in three hours and a half they assumed a green tint under the action of yellow light; whilst an hour longer was required for orange, and sixteen hours for blue. It is evident from this that the energy of solar action on plants corresponds neither with the maximum of heat, which lies in the red rays, nor in the maximum of chemical intensity which is at the other extremity of the spectrum, that is the violet.
If blades of grass are put into tubes filled with water charged with carbonic gas, and exposed to coloured rays, and the quantity of oxygen gas disengaged is measured, it will be found that the largest quantity is in the tubes which have been acted on by yellow and green light; afterwards those influenced by orange and red. Just as aquatic plants send out gaseous bubbles under white light, so do they to nearly the same extent under orange light, but twenty times less if placed under blue glass. These experiments would seem to prove that it is the luminous rays only, and principally the yellow and orange, that act upon plants. To this may be added, that green light produces much the same effect as darkness on vegetable respiration; thus explaining why there is such a slow lingering growth under the shade of large trees or forests, where the ground beneath is bathed in emerald light.
The sun also assists in the transpiration and constant renewal of the moisture essential to the tissues of plants. Like the human being, when there is no evaporation, the plant becomes dropsical, and the leaves fall because the stem is too weak to bear their weight. This imperious need and love which they have for light shews that the solar rays are really the essence which gives colour. The corollas of those flowers which grow on mountains at a great elevation have a deeper hue than those which blow in lowlands. The sun’s rays pass more easily through the transparent atmosphere which bathes the higher peaks. Certain flowers vary with their altitude; thus the Anthyllis vulneraria passes from white, through pale red, to an intense purple. Well-lighted and cleared tracts of land are much richer in colour than those shaded by high hedges and trees; and some flowers are observed to change during the day, owing to the direct action of the sun. The Hibiscus mutabilis, for instance, blooms white in the morning and becomes red at noon-day; the floral buds of the Agapanthus umbellatus are also white at early dawn and afterwards acquire a blue tint; the Cheiranthus camelea changes from white to lemon colour and then to a red violet. If a flower be taken as it is coming out of its sheath and wrapped in black paper, so as to intercept the light, it remains white; but recovers its colour when exposed to the sun. Nor are fruits any exception to this rule; the beneficial action of daylight is necessary to their development, and to all those principles which communicate taste and scent to the different parts.
Another part of this interesting study relates to the mechanical action which light exercises, as shewn in the sleep of flowers, the inflection of the stems, and the inclination towards the great luminary. Pliny speaks of the sunflower which always faces the sun and turns round with it; a delicate sensibility which the poet Moore has beautifully expressed in words and music. The lupine is another instance, which indicates by its diurnal revolution the hour of the day to the labourer. The stems of all plants as a rule turn towards the side of the light, and bend to drink it in. This constitutes what is known as ‘heliotropism.’ If cress be grown in darkness on moist cotton-wool, and then placed in a room lighted on one side only, the stems bend and incline very rapidly towards it; the higher part only turns, the lower remaining upright. But if it be placed in a room lighted by two windows, a fresh observation will be made. Supposing they are on the same side, and admitting an equal amount of light, the stem bends in the direction of the middle of the angle formed by the rays; whilst if one window allows more light to penetrate into the room than the other, the stem turns to it. When the two are opposite there is no deviation from the straight line.
There are some curious facts regarding climbing-plants; their stems generally turn from left to right round the pole used for support; others follow a contrary direction; while to some it seems to be a matter of indifference. Mr Darwin has concluded that light is an influential cause. If plants of this class are placed in a room near a window, the stem requires more time to perform the half-revolution during which it is turned away from the light, than for that which is towards the window. In one case the whole circle was completed in five hours and twenty minutes; of this the half in full light only required an hour; whilst the other could not traverse its part in less than four hours and twenty minutes—a very striking variation. Some Chinese ignamas, Diascorea batatas, in full growth were placed in a completely darkened cave, and others in a garden; in every case those which were in darkness lost the power of climbing round their supports; those exposed to the sun were twisting, but as soon as they were put in the cellar they grew with straight stems.
The sleep of plants, which certainly has a connection with light, is another curiosity in nature. Flowers and leaves of some growths seem to fade at particular hours, the corolla being closed, which after a state of lethargy blows out afresh; in others, the flower falls and dies without having closed. In the case of the convolvulus the flower is drawn up at noon. Linnæus noted the hours in which certain plants blow and fade, and thus composed a floral dial; but science has not yet been able to explain these curious relations to light.
The green colouring of leaves and stems is owing to a special matter called chlorophyll, which forms microscopical granulations contained in their cells. These grains are more or less numerous in each cell, and it is to their number as well as to the intensity of their colour that the plant owes its particular shade of green. Sometimes they are found pressed together and cover the whole internal surface of the cell; whilst at other times they are smaller in quantity and do not touch each other. It has recently been observed also in the latter case, that under the influence of light the green corpuscles undergo very curious changes of position; in certain plants they crowd to the part of the wall of the cells exposed to the action of the sun—a phenomenon which does not take place in darkness or under red rays only.
There might be given many other very interesting effects of light on plants, not usually noticed. The truth is, the direct rays of the sun exert a potent influence on every living thing, whether plant or animal. Sunlight, fair and full upon you and upon your dwelling, might be called the greatest blessing in nature; but on this branch of the subject we will not at present expatiate.