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 development, he was not in the habit of going back to the very
earliest and most instructive stages. Hence he failed to explain 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 ('Vermischte 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 characteristic, 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
