272 METAMORPHOSIS 



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 WIESNER. 1892. Die Elementarstruktur u. das Wachstum d. leb. Substanz. Wien. 

 WILDEMAN. 1893. Mem. couron. par 1'acad. Belg., 4to, 53. 

 WILLE. 1886. Ueber d. Entw. der Pollenkorner d. Angiospermen. Christiania. 

 ZACHARIAS. 1888. Bot. Ztg. 46, 33. 

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 ZIMMERMANN. 1893. Beitr. z. Morph. d. Pflanzenzelle, i, 209. 



LECTURE XXII 

 THE GROWING POINT 



THE daughter-cells which arise by cell division may either separate from 

 each other or they may remain connected. If they separate, the plant in 

 question is unicellular in the truest sense of the word, in the other case cell 

 aggregates arise in the form of cell filaments, cell plates and cell bodies according 

 as growth takes place in one, two or three dimensions of space. If all the cells 

 of such an aggregate are like each other, and each cell is physiologically quite 

 independent, the distinction between those which take the form of such aggre- 

 gates (colonies) and those which are strictly unicellular is not well marked, 

 and numerous transitions between these conditions are to be found ; indeed 

 one and the same plant may, according to external conditions, exhibit a uni- 

 cellular form or a colonial form. 



The two sister-cells are not, however, always morphologically and physio- 



