Theories of the Nutrition of Plants from Aristotle to Liebig *

Aristotle' had taught that the food of plants, like that of animals, was not simple, but composed of various combinations of the four elements-Earth, Air, Fire and Water. Plants, however, unlike animals, did not have organs for rendering their food suitable to their purposes, but took in nutriment peculiarly adapted to them. "For plants get their food from the earth by means of their roots; and this food is already elaborated when taken, which is the reason that plants produce no excrement, the earth and its heat serving them in the stead of a stomach." This concept that the plant took in food "already elaborated" was accepted for many centuries, and botanists, when they concerned themselves at all with the problems of plant nutrition, puzzled rather over the mechanism by which the "nutrient fluid" was circulated through the plant. The views of Cesalpino, whom Sachs calls "a faithful and gifted disciple of Aristotle", give us an insight into the physics of the day. In De Plarntis Libri XVI, 1583, he tried to explain what forces are responsible for the nourishment of plants.2 It cannot be the "ratio similitudinis" which draws iron to a magnet, or the roots would be drawn to the fluid of the earth, and not vice versa. Nor can it be the "ratio vacui", for since not only moisture but also air is contained in the earth, the plant would be filled not with juice but with air. Cesalpino suggested that a power of suction, "bibula natura", by which many dry things, such as a sponge, attract moisture, conveys the moisture continually from the earth into the plant. The great iatro-chemist, Van Helmont (1577-1644), approached the question of plant nutrition with an experimental attitude. Instead of accepting the view of Aristotle that the component parts of plants were introduced from the earth in a state ready for use, he attempted to prove, by a now classic experiment, that the whole substance of plants was formed from water alone, which he