230 The Microscope. 



ble, (Figs. 11-13), and about the time the segments reach the- 

 poles fine granules appear in the centre of the cell, forming a disk 

 at the equator in the place formerly occupied by the nuclear- 

 plate (Fig. 12). Careful examination shows these granules to 

 be thickenings of the connecting threads, and new threads are 

 found with similar thickenings at the edge of this '' cell-plate,"' 

 as it is called, which add to its size until it reaches completely 

 across the cell (Fig. 13), when the separate granules (" micros- 

 omes ") which appear to be cellulose, coalesce and form a con- 

 tinuous membrane dividing the cell into two (Fig. 14). 



In the meantime the nuclear filaments of the two daughter 

 nuclei, undergo in reverse order the changes that were observed 

 in the dividing nucleus, become invested with a membrane, the 

 nucleolus appears, and the new nucleus resembles in all essential 

 respects that from which it came. 



The complicated process (" Karyokinesis "), here sketched, we 

 find universal throughout the growing tissues of the higher 

 plants ; but as a rule, cells lose this power of division after as- 

 suming their permanent form. In a few instances known,, 

 among which may be cited the long internodal cells of the Cha- 

 raceas, and Iradescantia, a direct division (" Fragmentation ") of 

 the nucleus has been observed, resulting in the formation of 

 many nuclei without the complicated process of karyokinesis ;; 

 but, so far as known, this is confined to cells that have already 

 attained their growth, and never occurs in the actively growing 

 and dividing cells. 



It is not the object of this paper to discuss methods, but those 

 who desire to study the subject practically will find some notes 

 by the writer of this paper, published in the Bulletin of the 

 Torrey Botanical Club\ 



EXPLANATION OF PLATE III2. 



Fig. 1. — A filament of Nostoc showing stages of cell-division. The success- 

 ive stages indicated by the figures. 



Figs. 2-6. — A cell of Cladophora showing the process of cell-division. Fig^ 

 2 drnvm at 9:30 A. m. Fig. 6 at 2:05 p. m. 



Fig. 7. — A larger cell of the same plant. 



Fig. 8. — A dividing cell of Spirof/yra. 



Fig. 9. — A desniid in process of division. 



1 •• Studies in Cell-diTision." Bull. Torrey Bot. Club, Jan., 1890. 



2 For the use of the plate thanks are due the Editor of the Bulletin of the Torrejr 

 Botanical Club. 



