"The power of decomposing carbonic acid is a vital function which belongs to the leaves and bark. It has been stated, on the authority of Leibig, that during the night the plant acts only as a mere bundle of fibres—that it allows of the circulation of carbonic acid and its evaporation, unchanged. In his eagerness to support his chemical hypothesis of respiration, the able chemist neglected to enquire if this was absolutely correct. The healthy plant never ceases to decompose carbonic acid during one moment of its existence; but during the night, when the excitement of light is removed, and the plant reposes, its vital powers are at their minimum of action, and a much less quantity is decomposed than when a stimulating sun, by the action of its rays, is compelling the exertion of every vital function."
In hot, swampy countries, where vegetation is very rapid, and the soil loaded with decomposing carbonic matter, the plants absorb more carbonic acid than they require, and they then evolve carbonic acid gas from their leaves. Hence such climates as the West Indies are injurious to life, though favourable to vegetation.
1147. How do plants obtain hydrogen?
They obtain hydrogen in combination with oxygen in water, and with nitrogen, in the form of ammonia, as which it exists in animal manures.
1148. How do plants obtain nitrogen?
From the atmospheric air, and from the soil, in which it is combined with other elements.
1149. How do plants apply these elements to the formation of their own structures?
When those substances which form the food of plants are absorbed, either by their leaves or their roots, they are converted, with the aid of water, into a nutritive sap, which answers the same purposes in plants as blood does in animals.
1150. How is the nutritive sap applied to the growth and enlargement of the plant?