434 THE AMERICAN NATURALIST. [Vou XXXV I II. 



the thallophytes nuclear division may proceed through the entire 

 vegetative life of the organism without any segmentation of the 

 protoplasm which only takes place during the reproductive phase 

 of spore formation. But fundamentally protoplasmic segmenta- 

 tion depends on increase in the amount of protoplasm which 

 demands the multiplication of nuclei so that nuclear division 

 always precedes cell division, and we shall consider the events in 

 that order. 



(a) Events of Nuclear Division. 



i. Direct Division. 



The nucleus divides after one or two methods, either directly 

 by constriction or fragmentation, or indirectly (mitosis) when 

 there is present a fibrillar apparatus called the spindle. Direct 

 division is the only form present in the simplest plants and phy- 

 logenetically must have preceded the elaborate mechanism de- 

 manded for indirect division. This topic will be given especial 

 attention in Section VI '. Direct division is also present in cer- 

 tain specialized cells and tissues of higher plants. These are 

 generally old cells or tissues that are far removed from the gen- 

 eralized structure and potentialities of germ plasm. Yet some- 

 times direct and indirect division occur in the same cell, e. g., 

 Valonia (Fairchild, '94), and such forms might be made the 

 subject of very interesting investigations. In some cases the 

 phenomenon ,of direct nuclear division accompanies pathological 

 conditions or the degeneration of cells and may take the form of 

 extensive fragmentation. It would be outside of our purpose to 

 discuss such phenomena which is obviously abnormal, and the 

 primitive forms of nuclear division will be taken up later (Sec- 

 tion VI). It is possible that direct division in higher plants is in 

 a sense a reversion to early ancestral conditions, a reversion that 

 only comes on when for some reason the normal activities of the 

 germ cell are in abeyance or have ceased. 



2. Indirect Division (Mitosis). 



Indirect nuclear division, mitosis or karyokinesis, is character- 

 ized by a mechanism which varies greatly among plants in its 



