Food of Plants.

The simplest kind of nutrition is that presented to us by the vegetable kingdom, where water may be considered as the general vehicle of the nutriment received. Before the discoveries of modern chemistry it was very generally believed that plants could subsist on water alone; and Boyle and Van Helmont in particular endeavoured to establish by experiment the truth of this opinion. The latter of these physiologists planted a willow in a certain quantity of earth, the weight of which he had previously ascertained with great care; and during five years, he kept it moistened with rain water alone, which he imagined was perfectly pure. At the end of this period he found that the earth had scarcely diminished in weight, while the willow had grown into a tree, and had acquired an additional weight of one hundred and fifty pounds: whence he concluded that the water had been the only source of its nourishment. But it does not seem to have been at that time known that rain water always contains atmospheric air, and frequently also other substances, and that it cannot, therefore, be regarded as absolutely pure water: nor does it appear that any precautions were taken to ascertain that the water actually employed was wholly free from foreign matter, which it is easy to conceive it might have held in solution.