tubes are, he says on page 225, cylindrical or conical bodies
formed of spiral fibres which are afterwards surrounded by a delicate membrane. He puts annular, reticulated, and pitted
vessels together as metamorphosed spiral tubes. His explanation of these forms cannot well be understood except by
supposing that he assumed an actual metamorphosis in time
in accordance with the view of Rudolphi and Link ; but he
afterwards in his 'Neues System,' i. p. 140 declares this to be
a misunderstanding, though his real meaning is still doubtful; the obscurity attending the doctrine of metamorphosis did not
fail to cause misunderstandings in phytotomy, as it did in the
morphology of organs. Meyen makes only the striated and
pitted vessels in the wood convey air, the true spiral vessels
sap. That vessels are formed from cells, as Mirbel had already
maintained and Treviranus had partly observed, Meyen
intimates indeed, but not with an air of entire conviction.
The different forms of laticiferous organs are examined under the head of the ' system of circulation in plants.' Meyen sees in this system the highest product of the plant, being fully persuaded with Schulz, that the latex (milk), or as he also terms it the life-sap, is in constant circulation, like the blood in the veins. He gives a more summary account than is his wont of the course of the laticiferous organs, but bestows more care on the nature of the latex, and on the structure of the receptacles that contain it. That some of these are produced by cell-fusion, that others represent intercellular spaces, while others again are long branched cells, was not known to Meyen or even to later phytotomists before 1860.
This condensed account of the contents of Meyen's 'Phytotomie' shows a striking mixture of advance and retrogression, when compared with what had been achieved before his time; by the side of the fact established by Treviranus that the epidermis does not consist of a single membrane but of a layer of cells, to which Meyen assents, we find the gross mistake of taking the guard-cells of stomata for cuticular glands, the
