114 



The results of these investigations show that every nucleus (with 

 a few doubtful exceptions) arises from the division of a pre-exist- 

 ing one, and that among plants, at least, the origin of a nucleus 

 from undifferentiated protoplasm never occurs. 



In the simplest plants, like the Schizophytes, it is probable 

 that in most cases the nucleus substance is not concentrated in 

 the form of a definite nucleus, though such a body has been de- 

 monstrated in a number of the larger Cyanophycc(B, The latest 

 investigations of Zacharlas in this subject leave the question of 

 the presence of nuclei in these forms still open. In these 

 plants, especially those that are truly unicellular, or with cells 

 only slightly coherent, we find the simplest form of fission, a 

 mere constriction of the cell, without any participation of the nu- 

 clear substance beyond the mere mechanical separation of the 

 portions in the two daughter cells. 



Nostoc 



filaments in 



which all stages of cell-division are present, and the process is 

 easily followed. The resting cells, (i), are nearly globular. Pre- 

 vious to division they elongate in the direction of the axis of the 

 filament, (2), and a slight constriction appears in the middle of 

 the cell, (3). Later stages (4-6) show this constriction deeper, until 

 finally the daughter cells are completely separated by the for- 

 mation of a division wall, and becoming rounded off, assume the 



form of resting cells. 



The first plant in which the process of cell-division was care- 

 fully studied, and one of the best for showing simple fission, is 

 Cladophora. The common species are found in almost every 

 stream, and may be kept indefinitely in the laboratory. 



The cells are multi-nucleate, which may be shown by fixing 

 the cells with one per cent, chromic acid in water, thoroughly 

 washing and staining with borax-carmine or alum-cochineal. 

 The nuclear division is not easily shown, and takes place inde- 

 pendently of the cell-division. 



The latter may almost always be found by examining a lot of 

 actively-growing filaments, and when once found may be fol- 

 lowed through in the living cell. The later stages may be 

 quickly recognized, even when but slightly magnified, the green 

 contents of the cell being pushed away from the outer wall of the 



