312 Development of the Cell and [BOOK n. 



in which he announced this theory and set aside every objec- 

 tion that was made to it, combined with his great reputation at 

 the time, at once procured for it the consideration of botanists 

 generally ; and the most important representatives of phytotomy, 

 von Mohl himself at first not excepted, allowed that there was 

 a certain amount of justification for it. It was a question in 

 which theoretical considerations were not of primary import- 

 ance ; direct and varied observation of careful preparations 

 with strong magnifying powers could alone form the basis for 

 further investigation. Unger showed in this way that the pro- 

 cesses at the growing point of the stem could scarcely be 

 reconciled with Schleiden's theory, and in this view he was 

 supported by the English botanist Henfrey ; but Nageli was 

 the first who addressed himself with energy and sound reason- 

 ing to the important and difficult question, how cells are formed 

 in reproductive and growing vegetative organs, and how far the 

 processes are the same in the lower Cryptogams and in the 

 Phanerogams. He set out by assuming that Schleiden's theory 

 was in the main correct, but his long-continued investigations 

 led him finally to the conviction that it must be entirely 

 abandoned, and he proposed the outlines of the theory 

 of cell-formation which is accepted at the present time. In 

 this case, as before in questions of morphology, he applied 

 himself first, and with great success, to the investigation of the 

 lower Cryptogams, while Alexander Braun's observations on 

 some very simple Algae contributed materially to the further 

 development of the cell-theory, and especially to extending and 

 correcting the idea of the cell; Hofmeister's researches also in 

 embryology not only produced great results for morphology, 

 but at the same time supplied a variety of facts which served 

 to complete Nageli's view. The further this was worked out, 

 the more apparent it became that the external circumstances 

 in the processes of cell-formation might be very various, and 

 that von Mohl's earlier observations especially gave a correct re- 

 presentation of individual and typical cases ; but more important 



