Theory of the Nutrition [BOOK m.
from the stem, and by similar experiments ; whereas the simple consideration that it is only in the green leaves that carbonaceous vegetable substance is formed, would have made the existence of what was known as a descending sap appear to be a matter of course, and must have led to a much clearer conception of the matter. But this consideration was either quite overlooked or only mentioned incidentally by those who occupied themselves with experiments on the movement of the descending sap. This is the case in Heinrich Cotta's ' Natur- beobachtungen �ber die Bewegung und Function des Saftes in den Gewachsen,' 1806, in many respects an instructive work, and in Knight's otherwise serviceable experiments on the growth in thickness of trees. It was not till after 1830 that De Candolle and Dutrochet perceived that the fact that the green leaves are assimilating organs must be decisive of the question of the movement of the sap in the stem.
No progress was made with the general doctrine of nutrition between 1820 and 1840 except in one point, the absorption of oxygen by all parts of plants ; here something was done to consolidate the theory and to enrich it with new facts ; it was indeed a subject more adapted to the views of the day, because it at once suggested a variety of analogies with the respiration of animals. Grischow showed in 1819 that Fungi never de- compose carbon dioxide, but absorb oxygen and give off carbon dioxide. Marcet carried the subject further in 1834, after de Saussure had published in 1822 an excellent investigation into the absorption of oxygen by flowers ; in this work we have the basis laid for the theory of vegetable heat, to which we shall return. But Dutrochet was the first who made an elaborate comparison of the respiration of plants and animals (1837), and showed that not only growth, as de Saussure had already perceived, but also the sensitiveness of plants depends on the presence of oxygen, that is on their respiration. The recognition of the fact, that the inhalation of oxygen plays the same part in plants that it does in animals, prepared the way