64 



BOTANY 



PART I 



cessively, by means of diaphragm-like projections from the wall of the 

 mother cell. It was a division process of this kind (Figs. 60, 66), 

 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 wall of the 

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

 completely across it' (Figs. 60,. 66). In a division of this sort, in 

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

 wall is formed midway between the daughter nuclei (Fig. 65). In 

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

 the nuclear division does not differ from that of uninuclear cells, cell 

 division (Fig. 66) is altogether independent of nuclear division. And 

 in multinuclear, unicellular Thallophytes, nuclear division is not fol- 



M asi__^. 







Fig. 65.— Cell of Spirogyra in division, n, 

 One of the daughter nuclei ; w, developing 

 partition wall ; ch, chlorophyll band, 

 pushed inward by the newly-forming wall. 

 (X 230.) 



Fig. 66. — Portion of a dividing cell of 

 Cladoplwra fracta. w, Newly - forming 

 partition wall ; eft, intercepted chromato- 

 phore ; /:, nuclei. ( x 600.) 



lowed by a cell division. The interdependence of nuclear and cell 

 division in uninuclear cells is necessary to ensure a nucleus to each 

 daughter cell. In multinuclear cells it is not essential that cell 

 division should always be accompanied by nuclear division, as in any 

 case sufficient nuclei will be left to each daughter cell. 



Free Nuclear Division and Multicellular Formation. — The nuclear 

 division in the multinuclear cells of the Thallophytes may serve as an 

 example of free nuclear division, that is, of nuclear division unaccom- 

 panied by cell division. In plants with typical uninuclear cells, 

 examples of free nuclear division also occur ; although," in that case, 

 the nuelear division is customarily followed by cell division. This is 

 often the case in the formation of germ cells, and is due to the fact 

 that while the nuclei increase in number this process is not accom- 

 panied by a corresponding cell division. When, however, the number 

 of nuclei is completed, then the cytoplasm between the nuclei 



