Chap. iL] of Plants. Dc Candollc. 519 



to be conveyed chiefly through the intercellular passages. The 

 vessels probably share in certain cases in these functions, but 

 serve generally as air-canals. The cells appear to be the really 

 active organs in nutrition, since decomposition and assimila- 

 tion of the juices take place in them. Cyclosis (of Schultze's 

 vital sap l ) is a phenomenon which appears to be closely con- 

 nected only with the preparation of the milky juices, and to be 

 caused by the actively contractile nature of the cell-walls or of 

 the tubes. Woody and other substances are deposited in every 

 cell in different quantities according to their kinds and the 

 accompanying circumstances, and clothe their walls ; the 

 unequal thickness of the layer so deposited appears according 

 to Hugo von Mohl to have given rise to the supposition of per- 

 forated cells ; that is, the parts of the cell-wall that remain trans- 

 parent appear under the microscope as pores. Every cell may 

 be regarded as a body which prepares juices in its interior ; but 

 in vascular plants their activity stands in such a connection 

 with a complex of organs, that a single cell does not represent 

 the whole organism, as may be said of the cells of certain cellular 

 plants, which are all like one another. There is no circulation 

 in plants like the circulation in animals, but there is an alter- 

 nating ascent and descent of the crude sap and of the formative 

 sap which is often mixed with it. Both these phenomena depend 

 perhaps on the contractile power in cells that are still young, 

 and if so, this power would be the true vital energy in plants.' 



What is strange to us in De Candolle's theory of nutrition is 

 due chiefly to the predominance of the vital force ; yet at the 

 same time it gives the facts in their general connection, and its 

 best feature is, that the true function of the leaves, the decom- 

 position of carbon dioxide in light and the production of 

 organisable substance, is made the central point of the whole 

 circle of the processes of nutrition. Very different in this 

 respect were the views of the two most eminent German 



1 See above on page 5 1 3. 



