308 Examination of the Matured Framework [Book ii. 



stem of Monocotyledons, and at once did away with the whole 

 theory of endogenous growth in the opinion of all who were 

 capable of judging, though some even eminent systematists 

 for a long time maintained the old error. The results which 

 von Mohl obtained from his study of the comparative anatomy 

 of the stem, rested mainly on careful observation of the 

 mature tissue-masses, and when he studied the history of deve- 

 lopment, he was not in the habit of going back to the very 

 earliest and most instructive stages. Hence he failed to ex- 

 plain fully the real points of agreement and difference of 

 structure between Tree-ferns and other Vascular Cryptogams 

 and Phanerogams, and in like manner he stopped half-way when 

 engaged in explaining the secondary growth in thickness of 

 dicotyledonous stems from the nature of their vascular bundles, 

 and the formation of cambium. The account of growth in 

 thickness which he still gave in 1845 ('Yermischte Schriften,' 

 p. 153), and which rests less on observation than on an 

 ideal scheme, is highly obscure, and even in the treatise which 

 he published in the ' Botanische Zeitung' in 1858 on the 

 cambium-layer of the stem of Phanerogams, and in which he 

 criticises the newer doctrines of Schleiden and Schacht, the 

 subject is far from being fully cleared up, though the views 

 there advocated are decidedly superior to his former ones. A 

 satisfactory conclusion with respect to growth in thickness of 

 the woody body and of the rind was not reached till the history 

 of development in vegetable histology began to be more 

 thoroughly studied. 



As von Mohl had from the first laid special stress on the peculiar 

 character of the vascular bundles as compared with other tissue- 

 masses, so he perceived that the structure of the epidermis and 

 of the different forms of exterior tissue was thoroughly charac- 

 teristic, and he succeeded in arriving at a clearer understanding 

 of the matter in this case than in the other. Very confused 

 ideas had prevailed on the subject before he took it up, and we 

 owe to him the best and most important knowledge which we 



