CHAP, in.] of Cell-membrane in Plants. 265 



mentions Hedwig's notion already described, and shows that 

 its exact opposite is true, namely, that the spiral band is 

 surrounded by a membrane on the outside, a fact which was 

 afterwards denied by Link, Sprengel, and Moldenhawer. On 

 the other hand he did not understand the sculpturing on the 

 scalariform vessels ; he took the pits in the dotted vessels for 

 thickenings of the wall, such as are seen in the transverse ridges 

 between the slits in true scalariform vessels, and the slits he 

 thought were closed. If there was much that was erroneous 

 in these views, yet Bernhardi contributed essentially to the 

 clearing up of the subject by his effort to distinguish the 

 different forms of air-vessels, and especially by pointing atten- 

 tion to the fact that neither spiral nor annular vessels are found 

 in secondary wood. The resemblance between different forms 

 of vessels misled many of his contemporaries into supposing 

 that they are due to metamorphosis of true spiral vessels. 

 Bernhardi showed that different forms of wall are found inside 

 one vascular tube, but that this does not depend on modifica- 

 tion with age ; observation rather teaches that every kind of 

 vessel receives its character in its young state, and especially 

 that the youngest scalariform vessels do not present the form 

 of spiral vessels. 



Under the head of vessels proper he reckoned all tubular 

 forms filled with a peculiar juice, milk-cells and true milk-vessels, 

 and also resin-ducts and the like, and he made many good and 

 still valuable observations on their distribution and sap-contents. 

 He could not see the differences of structure in these various 

 fluid-conveying vessels with the low magnifying power of his 

 glass, and therefore attended chiefly to the structure of the 

 large resin-ducts, which on the whole he rightly understood. 



The question whether there are any other forms of vessels in 

 the plant beside those already named gave him occasion to 

 define a vessel better than it had yet been defined, namely as 

 an uninterrupted tube or canal, and at the same time he found 

 himself obliged to consider whether his bast-threads are vessels ; 



