No. 450.] STUDIES ON THE PLANT CELL. 449 



mitosis emphasize the importance of the chromosomes which 

 are really the only enduring structures in the nucleus and have 

 led to their being considered as the probable bearers of heredi- 

 tary qualities. 



3. The Dynamics of Nuclear Division. 



Mitotic phenomena in certain plant cells present evidence that 

 has very direct bearing on some of the theories that deal with 

 mechanical and dynamical explanations of nuclear division. The 

 methods of spindle formation and the various forms of kinoplas- 

 mic structures (centrosomes, centrospheres and kinoplasmic 

 caps) which generally in plants seem not to be permanent 

 organs of the cells all tend to support Strasburger's conception 

 of kinoplasm, which is an outgrowth and application to plants of 

 Boveri's well known theory of archoplasm. 



The centrosome theory is supported by very few investiga- 

 tions in Botany, the most notable being that of Swingle ('97), 

 for Stypocaulon, who believes that the centrosome divides with 

 the aster and is maintained as a permanent organ throughout 

 successive cell divisions. Other examples of similar conditions 

 may be found among the thallophytes which, after all, have 

 received very little attention, and such types as Dictyota and 

 the diatoms offer excellent subjects for studies covering a 

 series of cell divisions. But in contrast to Stypocaulon it should 

 be noted that the conspicuous centrospheres of Fucus and Cor- 

 allina disappear with each mitosis to be formed anew, and the 

 same conditions obtain in the germinating spores of liverworts 

 (Pellia). There seems to be no place for the centrosome in 

 spindle formation as presented in the spore mother cells of all 

 groups above the thallophytes (see Sec. III). Neither does 

 mitosis in the vegetative tissues of these groups, characterized 

 as it is by the presence of kinoplasmic caps, conform to the 

 program of the centrosome theory. 



The morphological manifestations of kinoplasm are so various 

 that we are driven to a very general conception of its organiza- 

 tion. Kinoplasm runs through cycles in which the structure 

 passes from a granular condition to a fibrillar and then back again 



