CHAP, ii.] of Plants. Boussingault. 533 



chlorophyll ; further thai 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 after- 

 wards 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 con- 

 fined 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 experi- 

 ments 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 



