290 Examination of the Matured Framework [BOOKII. 



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 ex- 

 planation 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 ' Phy- 

 totomie ' 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 



