274 Examination of the Matured Framework [BOOK n. 



if we suppose with Mirbel that the dividing wall is single ; he 

 is only concerned to enquire whether the perforated projections 

 lie on the one or on the other side of the wall. He refers 

 Treviranus, who had denied the presence of the pores, to his 

 description of scalariform vessels, in which he had himself 

 seen the slits which correspond to the pores. 



In comparison with these fundamental questions Mirbel's 

 further account of matters of detail does not concern us here. 

 He gave a connected view of the whole of his doctrine of 

 tissues in the form of aphorisms, which occupy the second 

 part of his book. Of all that he says on the five kinds into 

 which he distinguishes vessels the most interesting is the 

 statement, that diaphragms pierced like a sieve separate the 

 different members of his 'beaded' vessels. We find that the 

 weakest part of Mirbel's phytotomy, as of that of his opponents, 

 is his description of the true vessels (vasa propria), with which 

 he classes the milk-cells of the Euphorbiae and the resin-ducts 

 of Coniferae, but he saw clearly enough that the latter were 

 canals inclosed in a layer of tissue of their own. The third 

 part of the book is devoted to these forms of tissue, and we 

 learn that he classes not only many kinds of sieve-cell-bundles, 

 but also true bast- fibres, as those of nettle and hemp, with his 

 bundles of true vessels. Like his opponents he makes the 

 growth in thickness in woody stems to be due to transformation 

 of the inner layer of bast ; but he gives a new turn to this 

 view, which brings it nearer to the- modern theory; during the 

 period of vegetation a delicate tissue with large vessels is 

 developed in Dicotyledons on the confines of the wood and 

 the bark, and these augment the mass of the woody body, 

 while a loose cellular tissue is formed on the other side, 

 destined to replace the constant losses of the outer rind. To 

 later phytotomists, who understood by the word cambium a 

 thin layer of tissue constantly engaged in producing wood and 

 rind, Mirbel's otherwise indistinct conception of growth in 

 thickness must have become more indistinct from his using 



