STUDIES ON THE PLANT CELL. — V. 



BRADLEY MOORE DAVIS. 



Section IV. Cell Unions and Nuclear Fusions in 



Plants. 



The forms of cell unions and nuclear fusions in plants fall 

 into two groups : (i) those which obviously have no sexual sig- 

 nificance attached to them, and (2) those which are evidently 

 sexual acts. But apart from these simple divisions there are 

 some very interesting conditions in which it is far from easy to 

 determine whether certain events have a sexual significance either 

 physiologically or phylogenetically. The real test of such prob- 

 lems should lie in the evolutionary history of the processes 

 involved, for every sexual condition in plants has probably 

 developed in obedience to the same physiological demands and 

 in an essentially similar manner. However, we cannot apply the 

 evolutionary test in many cases where we have little evidence of 

 the developmental history of the group and such forms must rest 

 for the present as unsolved problems. We shall treat them in 

 special connections later in the paper. 



The material of this section will be presented under the fol- 

 lowing heads : — 



1. Protoplasmic connections between cells (plasmodesmen). 



2. Sexual cell unions and nuclear fusions. 



3. Asexual cell unions and nuclear fusions. 



I. Protoplasmic Connections between Cells (Plasmodesmen). 



It has been known for a great many years that the walls 

 between the cells in some plant tissues and more especially 

 between the cells of filaments in certain thallophytes were 

 crossed by deilicate strands of protoplasm so that contiguous pro- 

 toplasts were not entirely separated from one another. This fact 



317 



