CHAP, in.] of Ccll-incmbranc in Plants. 297

development of phytotomy we can distinguish satisfactorily two periods in his scientific career, the first of which extends from 1827 to about 1845. Before 1845 he was acknowledged to be the first of phytotomists, decidedly superior to all rivals ; his authority, though often attacked by unimportant persons, grew from year to year. This period may be said to close with the publication of his ' Vermischte Schriften ' in 1845. Up to that time investigations into the form of the solid framework of cell- membrane had chiefly attracted the interest of phytotomists, and in this subject there was no one who could measure him- self with von Mohl. Yet he began soon after 1830 to study the history of development in plants; in 1833 he described the development of spores in a great variety of Cryptogams, in 1835 the multiplication of cells by division in an alga, and the cell-division in the formation of stomata in 1838 ; in this period appeared Mirbel's first observations on the formation of pollen- cells (1833). Von Mohl too was the first, if we disregard Treviranus' somewhat imperfect notices of the origin of vessels in 1 806 and 1 8 1 1, who explained the history of the development of those organs ; and his theory of the thickening of cell-mem- branes, the principles of which are to be found in his treatise on the pores in cellular tissue (1828), may also be regarded as a mode of conceiving the sculpture of the cell-membrane from the point of view of the history of development.

Ever since 1838 Schleiden had raised the history of develop- ment to the first rank in botanical investigation, but he had proposed a thoroughly faulty theory of cell-formation, to which von Mohl at first at least did not withhold his assent in spite of previous and much better observations ; but after 1842 Niigeli devoted himself still more thoroughly and with more lasting results to the study of the development both of vegetable cells and tissue-systems, and of the external organs. He introduced new elements into phytotomic research, and it soon became apparent that even the questions hitherto examined must be

grappled with in a different fashion. Von Mohl did not hold aloof