90 



BOTANY 



PART I 



first investigated in fresh- water Algae, that gave rise to the conception 

 of cell division, which for a long time prevailed in both animal and 

 vegetable histology. In this form of cell division the new wall com- 

 mences as a ring-like projection from the inside of the Avail of the 

 mother cell, and gradually pushing farther into the cell, finally extends 

 completely across it (Figs. 94, 95). In a division of this sort, in 

 uninucleate cells, nuclear division precedes cell division, and the new 

 wall is formed midway between the daughter nuclei (Fig. 94). In the 

 multinucleate cells of the Thallophytes, on the other hand, although the 

 nuclear division does not differ from that of uninucleate cells, cell 

 division (Fig. 95) is altogether independent of nuclear division. And 



w 



fJh 



Fio. 94. — Cell of S])(ro(jyi-a in division, w 

 One of the daughter nuclei ; u; dovelop- 

 in;^ partition wall : ch, cliloroplast 

 pushed inward by the newly forming 

 wall. (X 230.) 



I 



Fio. 95.— Portion of a dividing cell of Clado- 

 phora fracta. w, Newly forming pattition 

 wall ; ch, dividing chromatophore ; k, 

 nuclei, (x 600.) 



in multinucleate, unicellular Thallophytes, nuclear division is not 

 followed by a cell division. 



The interdependence of nuclear and cell division in uninucleate cells 

 is necessary to ensure a nucleus to each daughter cell. In multi- 

 nucleate cells it is not essential that cell division should always be 

 accompanied by nuclear division, as in any case a sufficient number 

 of nuclei will be left to each daughter cell. 



Free Nuclear Division and Multicellular Formation. — The nuclear division in 



the imiltinuclcate cells of the Thallophytes may serve as an example of free nuclear 

 division, that is, of nuclear division unaccompanied by cell division. In plants 

 with typical uninucleate cells, examples of free nuclear division also occur ; although, 

 in that case, the nuclear division is customarily followed by cell division. "While 

 tiie nuclei increase in number by repeated division, this process is not accompanied 

 by a corresponding cell division. When, however, the number of nuclei is coni- 

 ])leted, the cytojilasm between the nuclei divides simultaneously into as many 

 portions as there are nuclei. In this process we have an example of multicellular 

 formation. This method of development is especially instructive in the embryo-sac 

 of Plianerogams, a cell, often of remarkal)le size and rapid growth, in which the 

 future embryo is developed. The nucleus of the ra2)idly growing embryo-sac 



