EFFECTS OF LIGHT AND AIR ON PLANTS.
The importance of light and air to plants is well known. When unassisted by these agents, plants lose their colour, and are deprived of many of their properties. Colour is thus evidently produced by the absorption of carbonic acid gas: and the colouring matter may be detected by a powerful microscope, lodged in the cellular substance of the leaf. How this colour is formed, and why it assumes different tints in different plants, are, however, questions which it is at present impossible to decide. The secretions of plants depend upon light, and their flavour and nutritious qualities are materially altered by their exclusion from it. The importance of this knowledge to a practical horticulturist is proved by the fact, that sea-kale, so well known as a wholesome and palatable vegetable, is not eatable in its original state; and that any part of the cultivated plant, if accidentally left exposed to the action of the air and light, becomes tough, and so strong in flavour as to be extremely unpleasant to the taste. Celery, also, in its native state, is poisonous; and it is only the parts that are blanched that are perfectly fitted for the table. Though colour is generally supposed to depend principally on the plant's being exposed to the light, some portion of colouring matter appears to be occasionally absorbed by the root. This colouring substance is, however, never a deep green. Red and yellow, as may be seen in forced rhubarb, &c., are the most common hues. Succulent plants are less susceptible of the influence of light than any others. As they are always natives of hot countries, nature, to prevent the danger they would be exposed to from excessive evaporation, has provided them with leaves almost destitute of pores; and the moisture they absorb by their roots thus remains for the nourishment of the plant. It is for this reason that cactuses, mesembryanthemums, and other plants of a similar description, require very little water when kept in pots. Scarcely any carbon is found in plants grown in the dark. Many experiments have been tried to show the stimulus afforded to vegetation by light; trees of the same species and variety have been planted in the same garden and the same soil, but against walls with different aspects, and differently situated with regard to shade. The effect has been, not only a difference in the growth and appearance of the tree, but also in the size, colour, and flavour of the fruit which it produced. The contrast between plants grown in hot-houses with wooden sash frames, and those grown in hot-houses with iron sash-frames, has been found equally striking; the difference of light between the two kinds of houses being as seven to twenty-seven, or, sometimes, as three to twenty-three. Light is required at an early period of vegetation; but, as its properties are to give strength and flavour, it must be admitted with caution, as it is sometimes injurious. Too much light renders the skin of fruits tough, and will make cucumbers bitter. Bérard of Montpelier found that the ripening of fruits is merely the turning the acid which they contain into sugar, by exposure to the light; and that too much light and heat, before they have attained their proper size, will bring on premature ripening, and make them insipid.
Lindley's Lectures, reported in the Gardeners' Magazine.