farr: cytokinesis of pollen-mother-cells 311 



the actual force exerted by d would be less than EB. As e ap- 

 proaches d this force would be much greater, whereas the force 

 resulting from the attraction of a, b, and c would diminish. The 

 resultant of forces in the early part of the process would be about 

 TfEB. But in a binucleate cell the resultant of forces which would 

 operate to draw the plasma membrane along the equatorial plane 

 would be the sum of the attracting forces of only two nuclei, each 

 in such a relation as b to e. It thus appears that the resultant of 

 forces in a binucleate cell w^ould be less than those in a tetra- 

 nucleate cell. 



When the furrow is formed, the nucleus is more nearly enveloped 

 by the plasma membrane and hence the position of equilibrium, 

 on the basis of the electrical charges postulated above, would 

 be nearer the center of the lobes. And when constriction is com- 

 plete we should find the nucleus in the exact center of the daughter 

 cell. It is to be borne in mind, of course, that the furrows as they 

 come nearer and nearer together, being of like sign will repel each 

 other. Also the opposite surfaces of the same furrow as it grows 

 deeper and deeper will more and more repel each other. The 

 problem almost immediately becomes too complex for analysis 

 in the present condition of our knowledge as to the actual distri- 

 bution of the ions in the cell solutions and on its membranes. I 

 have merely endeavored to point out certain possibilities as to the 

 distribution of the forces concerned in initiating cell-division by 

 the process of constriction. While it is not contended that these 

 electrical charges are the only factors in the division of the cells, 

 yet it is interesting to note that both the nuclear and plasma mem- 

 branes behave in the initial stages of divi sion as if so charged. 



It is quite likely that the thickened mother- wall exerts no un- 

 evenly distributed force of compression upon the protoplast while 

 in the spherical form. The tetrahedral arrangement of the spores 

 has been shown to have originated in the migration of the nuclei 

 when the mother-cell was spherical in form, hence it is not neces- 

 sary to postulate the cell-wall as the determining factor in the 

 accomodation to least surfaces, which is effected by the tetra- 

 hedral arrangement. Furthermore, the surface tension of the 

 protoplast may be well thought to operate to bring about this 

 result. The mother-wall seems to behave as if it were a plastic, 

 more or less gelatinous, substance with some elasticity, so that it 



