CHAP, ii.] of Plants. DC Candolle. 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 1 ) 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 513.