CHAP, in.] of Cell-membrane 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 1806 and 181 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 Nageli 

 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 



