CHAP, ii.] of Plants. Dutrochet. 511 



there must also be a corresponding exosmose at the roots ; 

 and this, which was called root-discharge, Macaire Prinsep 

 thought he had actually discovered, and even Liebig firmly 

 believed in its existence till a recent period, although the 

 researches of Wiegman and Polstorff (1842) and later more 

 careful investigations showed, that there was no noticeable 

 discharge by exosmose to answer to the great quantity of 

 water with substances in solution in it which is taken up by 

 the roots. Again, Dutrochet's theory of endosmose did not 

 fully explain the way in which the several substances which 

 feed the plant find their way into and are disseminated in 

 it. But notwithstanding these and other defects it deserved 

 the greatest consideration, because it gave the first impulse 

 to the further development of the theory of diffusion, and 

 contained a mechanical principle which might serve to explain 

 very various phenomena in vegetation as yet unexplained. 

 Dutrochet hastened to apply it to this purpose, where it was at 

 all possible to do so, and chiefly in his treatise on the ascend- 

 ing and descending sap ('Memoires,' 1837, i. p. 365), which 

 was superior to anything which had been written on the move- 

 ment of the sap in plants in its clear conception of the question 

 and in perspicuity of treatment. It should be especially men- 

 tioned that Dutrochet formed a true estimate of the functions 

 of the leaves as regards both the ascending and descending 

 sap, and to some extent pointed out the fault which lies at the 

 bottom of the earlier experiments with coloured fluids. After 

 communicating a number of good observations on the paths of 

 the ascending and descending sap, and noticing particularly 

 that in the vine the vessels of the wood serve for the movement 

 of the sap only in spring, when vines bleed, but that they are 

 air-passages in summer, when transpiration causes the most 

 copious flow of water in the wood, he proceeds to consider the 

 forces which effect the movement of the ascending sap in the 

 wood both in spring and summer. He first of all judiciously 

 distinguishes two things which had been before always mixed 



