300 Examination of the Matured Framework [BOOKII. 



afterwards there are large vessels, perfectly closed large cylin- 

 drical tubes with a transparent and very delicate membrane.' 

 He then shows how by degrees the sculpture peculiar to the 

 walls of vessels is formed on the inside of these tubes, and he 

 takes the opportunity of saying that a metamorphosis in time 

 from one form of vessel into another is entirely out of the 

 question, as Treviranus also and Bernhardi had maintained. 'The 

 dividing walls (transverse septa),' he continues, ' are formed in 

 a precisely similar manner to the side-walls of vessels ; only the 

 original tender membrane of the septa is usually lost in the 

 meshes of the network of fibres.' From that time no phytoto- 

 mist capable of an independent judgment has had any doubt 

 with regard to this view of the formation of the vessels in wood. 

 It is however striking enough that von Mohl, who thought it so 

 important to show that the cell is the sole foundation of veget- 

 able structure, never extended the proof to milk-vessels and 

 other secretion-canals in order to show whether and how these 

 also are formed from the cells. In his treatise on the vegetable 

 cell (1851) he still expressed doubt about Unger's assertion, 

 that the milk-vessels are also formed from rows of cells that 

 coalesce with one another, and held rather to the view of 

 an anonymous writer in the ' Botanische Zeitung ' of 1846, 

 page 833, that these vessels are membranous linings of gaps 

 in the cell-tissue. He might well lose his taste for the exam- 

 ination of these and similar organs after Schultz Schultzenstein 

 had by his various treatises, written after 1824, on the so-called 

 vital sap and the circulation which he attributed to it, made 

 this part of phytotomy a very quagmire of error, and had not 

 refrained from replying in an unbecoming manner to von Mohl, 

 who repeatedly opposed his views ; moreover Schultz's essay 

 'Ueber die Circulation des Lebenssafter ' (1833), which teems 

 with absurdities, had received a prize from the Academy of Paris. 

 2. The growth in thickness of the cell-membrane, and the 

 sculpture caused by it was a subject that is more or less con- 

 nected with most of von Mohl's writings. He developed the 



