CHAP, ii.] of Plants. DC Saussure. 505

where questions of nutrition turned on the movement of the sap, was the backward condition of the study of the inner structure of plants, as described in the second book. For instance, the question of the descending sap was complicated in the strangest way by Du Petit-Thouars's theory of bud-roots that descend between the bark and the wood ; Reichel's un- founded idea of the rising of the sap in the tubes of the wood was generally accepted, and a still worse error was maintained by some, that the intercellular spaces of the parenchyma are true sap-conveying organs. In 1812 Moldenhawer had to in- sist, but without producing any general conviction, that the vessels of the wood contain air, and Treviranus in 1821 that the stomata serve for the entrance and exit of air. We need not notice here what nature-philosophers like Kieser said about nutrition and the movement of the sap ; but even those who were far from adopting the extravagancies of this school were incapable of either making use of or carrying on the labours of Ingen-Houss, Senebier, and de Saussure. We may adduce in proof of this statement the remarks of Link on the function of leaves in his 'Grundlehren der Anatomic und Physiologic,' 1807. He says at p. 202 that their function is according to Hales transpiration, according to Bonnet absorption, according to Bjerkander the exudation and secretion of a variety of fluid.-,. according to Hedwig the storing up of juices, and inasmuch as leaves increase the green surfaces of plants, bear stomata and hairs, and hold a quantity of juices in their abundant paren- chyma, we may ascribe all these functions, but none of them exclusively, to leaves ; the only thing peculiar to them is that they convey elaborated juices to the young parts. Their great work, the decomposition of carbon dioxide, he does not nun tion. But this neglect of the doctrines of Ingen-Houss, Senc bier, and de Saussure was common, especially in Germany : it is seen in the efforts made to prove once more the existence of a descending sap in the rind, just as it had been proved in the

two previous centuries, by the result of removing a ring of bark