Page:History of botany (Sachs; Garnsey).djvu/553

] chlorophyll; further that the nitrogen which plants assimilate is derived from ammoniacal salts or nitrates, and that the alkalies, alkaline earths in the form of sulphates and phosphates, are indispensable ingredients in the food of plants, must be considered to be the great results of the labour bestowed on the theory of nutrition in the period from 1840 to 1860, while the way was also prepared for many points, which were afterwards of the first importance in the enquiry.

On the other hand the advance made in the theory of the movement of the sap from the time of Dutrochet till nearly 1860 was so small as to be scarcely worth mentioning ; yet it was an advance, that the physiological value of the doctrine of endosmose was more and more highly estimated, and that more solid proofs of the theory itself and a more exact acquaintance with osmotic processes were making it possible to explain more of the details of the movement of material in the plant, though the whole question was far from being finally settled. One discovery must be specially mentioned, the establishment by Hofmeister in 1857 of the fact, that the phenomenon observed for centuries in the grape-vine and other trees, and more recently in Agave and in many tropical climbing plants, known by the name of bleeding or weeping and supposed to be confined to certain periods of vegetation, not only occurs in all plants with true woody cells, but may be produced in them at all times by suitable means. The knowledge of this fact was an aid to the investigation of the cause of the weeping.

The theory of the descending sap was in the least advanced condition during this period; appeal was still made to experiments of the kind which Malpighi, Du Hamel, and Cotta had made, and which in reality show nothing more than that in dicotyledonous woody plants a food elaborated in the leaves is carried downwards through the cortex. As soon as it was understood, that all organic substance originates in the leaves, a fact which no one could doubt after 1840, no experiment was required to prove that the formative matter necessary for