BOOK ii.] Introduction. 227 



single form ; it attracted great attention in the botanical world, 

 but could not easily be reconciled with what was already 

 known; and in 1846 it was completely refuted by Na'geli, 

 who substituted for it the history of the formation of the 

 various kinds of vegetable cells in their main features, based 

 on profound and extensive investigations. It was natural that 

 these researches into the formation of cells should turn the 

 attention of observers, which had hitherto been almost ex- 

 clusively devoted to the solid framework of cell-tissue, to the 

 juicy contents of cells. Robert Brown had already discovered 

 the cell-nucleus ; Schleiden recognised its more constant pre- 

 sence, but misunderstood its relation to cell-formation ; Na'geli 

 and von Mohl next demonstrated the peculiar nature of proto- 

 plasm, the most important component of vegetable cells, and 

 especially the weighty part which it plays in their origination. 

 Unger in 1855 called attention to the great resemblance which 

 exists between the protoplasm of the vegetable cell and the 

 sarkode of the more s-imple animals, a discovery which was 

 subsequently brought into prominence by observations on the 

 behaviour of the Myxomycetes, and after 1860 finally led 

 zootomists as well as phytotomists to the conclusion, that proto- 

 plasm is the foundation of all organic development, vegetable 

 and animal. But there is yet another direction in which the 

 study of the history of development by the phytotomist 

 led to new points of view and to new results ; we have 

 already pointed in the end of the first book to the way in 

 which Na'geli after 1844 made the sequences of cell-division in 

 the growth of organs the basis for his morphology, and how 

 in this way the Cryptogams especially revealed their inner 

 structure; we also noticed the splendid results which Hofmeister 

 achieved by his study of the development of the embryo ; 

 here we have further to show, how after 1850 the various forms 

 of tissue, especially the vascular bundles, \\viv examined by 

 observation of the history of their development, and how 

 in this way botanical science has succeeded in explaining the 



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