40 GROWTH AND CELL-DIVISION 



a corresponding reduction of the nucleus. The size of the latter, however, 

 varies to a relatively small extent 1 , and it is in correspondence with this fact 

 that the nucleus forms so large a portion of the bulk of a small embryonic 

 cell 2 . 



SECTION 12. The Eelation between Cellular and Nuclear Division 3 . 



These two phenomena, though closely related ones, do not always 

 follow one another in the same regular sequence either as regards time 

 or place. A separation between them occurs normally, for example, in 

 Char a and Cladophora^, in laticiferous cells, and may also be induced in 

 other plants by special external agencies. That the nucleus should be able 

 to divide independently of the cytoplasm is no more surprising than that 

 the chloroplastids should be able to increase in number by fission without 

 cell-division necessarily occurring. Where, however, a cell contains a single 

 chloroplastid or a small definite number of them, their division precedes 

 cell-division, just as does nuclear division in uninucleate cells. It is, 

 therefore, impossible to say whether the mechanical aid of the cytoplasm 

 is or is not necessary in such cases for the division of the nucleus 5 . 



The final work of separation is performed by the cytoplasm, as is very 

 clearly shown in Spirogyra and Cladophora^ and in gymnoplasts it does not 

 involve the interposition of a cell-plate or dividing membrane between the 

 two segments 6 . Nor need the formation of the cell-plate necessarily 

 always take place in the same manner. 



Under special external conditions it may be possible to discriminate between 

 the above factors. Thus, according to Demoor 7 , low temperatures, a deficiency 



1 On the changes of size in nuclei see Schwarz, Conn's Beitrage, 1892, Bd. V, p. 80; 

 Zacharias, Flora, 1895, Erg.-bd., p. 217; Strasburger, 1893, I.e., p. 117. 



3 These relative sizes are the result of definite directive agencies, which also determine the 

 enlargement of the vacuoles and the reduction of the protoplasm to a peripheral film. The latter, 

 therefore, are not the primary causes inducing a cessation of cell-division. The bulk of the 

 protoplasm is, however, one factor in inducing division, and hence an inequilateral distribution of 

 the protoplasm, nucleus, or nuclei may be the immediate cause of an unequal division. Cf. 

 O. Hertwig, Zellen u. Gewebe, 1895, p. 180; Driesch, Ergeb. d. Anat. u. Entwickelungsgesch. von 

 Merkel u. Bonnet, 1898, Bd. vni, p. 749. Mottier (Annals of Bot., 1899, Vol. xiu, p. 358) 

 produced the unequal distribution of the cell-contents by means of centrifugal force. 



3 For details see Zimmermann, Morphol. u. Physiol. des pflanzlichen Zellkerns, 1896; 

 O. Hertwig, Zellen u. Gewebe, 1893 ; Delage, La structure du protoplasma et rh^redite", 1895. 

 Further, Strasburger, Jahrb. f. wiss. Bot., 1897, Bd. xxx, p. 155, and 1898, Bd. xxxi, p. 511; 

 Histologische Beitrage, 1900, Heft 6, and the works there quoted. None of the speculations upon 

 the mechanics of division is based upon a sure foundation (Chap. xv). 



4 Cf. e.g. Strasburger, Histol. Beitrage, 1893, Heft 5, p. 108. 



5 Cf. R. Hertwig, Abhandl. d. Miinchner Akad., 1898, Bd. xix, p. 698. 



6 In regard to animal cells see also Hoffmann, Bot. Ztg., 1898, Ref., p. 214. On the 

 influence of artificial division on cell- wall formation see Townsend, Jahrb. f. wiss. Bot., 1897, 

 Bd. xxx, p. 484. 



7 Demoor, L'e"tude de la physiol. de la cellule, 1894, p. 30 (repr. from Archiv. d. Biol., 



