272 Examination of the Matured Framework [BOOKII. 



transverse bands on false spiral vessels (scalariform ducts) and 

 the pits of dotted vessels are formed on the walls of membranous 

 fibre-tubes ; in like manner he derives true spiral vessels from 

 long thin-walled cells, on whose inner surface the spiral band 

 is formed, and well compares the members of young spiral 

 vessels with the elaters of the Jungermannieae. Here then we 

 find the first more definite indications of a theory of growth in 

 thickness of cell-walls, which, like the theory of the origin of 

 vessels from rows of cells, was afterwards developed by von Mohl 

 and laid on better foundations. At the close of the essay the 

 histology of the Cryptogams, Monocotyledons and Dicotyledons 

 is compared, and the subject is better and more perspicuously 

 handled than in the corresponding chapters of his competitors. 

 Though Treviranus' account of vegetable tissues was on the 

 whole weak as far as concerns the history of development, yet 

 MiRBEL 1 recognised in him the most dangerous opponent of 

 his own theory, and addressed a public letter to him and not 

 to his other German antagonists, Sprengel, Link and Rudolphi, 

 in defence of the views he had formerly expressed. This 

 letter is the first part of a larger work which appeared in 1808, 



1 Charles Frar^ois Mirbel (Brisseau-Mirbel) was born at Paris in 1776, 

 and died in 1854. He began life as a painter, but having been introduced 

 by Desfontaines to the study of botany, he became Member of the Institute 

 in 1808, and soon after Professor in the University of Paris. From 1816 to 

 1825 the cares of administration withdrew him from his botanical studies, 

 but he resumed them and became in 1829 Professeur des cultures in the 

 Museum of Natural History. Mirbel was the founder of microscopic veget- 

 able anatomy in France. All that had been accomplished there before his 

 time was still more unimportant than the work done in Germany. His 

 writings involved him in many controversies, and he made enemies by 

 refusing in his capacity of teacher to allow the importance at that time at- 

 tributed to systematic botany, but directed his pupils to the study of struc- 

 ture and the phenomena of life in plants. We are told by Milne-Edwards 

 that he suffered much from the fierce attacks which were made upon him ; 

 he sank at last into a weak and apathetic state, and was for some time 

 before his death unable to continue his studies or official duties ('Botanische 

 Zeitung' for 1855, p. 343). 



