506 Theory of the Nutrition [BOOK in. 



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 iiber 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 



