CHAPTER II. 



NUTRITION IN VEGETABLES. 



1. 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 be- 

 lieved 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 di- 

 minished 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 con- 

 tains 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. In an experiment of Duhamel, on the 

 other hand, a hcrsc-chesnut tree and an oak, exposed to the 

 open air, and watered with distilled water alone, the former 

 for three, and the latter for eight years, were kept alive, in- 



