Page:History of botany (Sachs; Garnsey).djvu/513

] succeeded in setting himself quite free from the old notions, and developed his antiphlogistic system into a connected whole. It should be mentioned that he had discovered in 1777 that the respiration of animals is a process of oxidation which produces their internal heat, heat being the product of every form of combustion. This fact was equally important for vegetable physiology, but it was some time before it was used to explain the life of plants.

The establishment of the fact, that parts of plants give off oxygen under certain circumstances, did little or nothing to further the theory of their nutrition ; and that was all that vegetable physiology owes to Priestley. Ingen-Houss on the other hand determined the conditions under which oxygen is given off, and further showed that all parts of plants are constantly giving rise to carbon dioxide ; on these facts rests the modern theory of the nutrition and respiration of plants, and we must therefore consider that Ingen-Houss was the founder of that theory. But since we are dealing here with a discovery of more than ordinary importance, it seems necessary to go more closely into the details.

A work of Priestley's appeared in 1779, which was translated into German in the following year under the title, 'Versuche und Beobachtungen über verschiedene Theile der Naturlehre,' and contained among other things the writer's experiments on plants. His way of managing them was eminently unsuitable, nor did he arrive at any definite and important result, though he expressed the idea which had led him to make them clearly enough, where he says, 'If the air exhaled by the plant is of better character (richer in oxygen) than atmospheric air, it follows that the phlogiston of the air is retained in the plant