STUDIES ON THE PLANT CELL. —.V. 
BRADLEY MOORE DAVIS. 
Section IV. CELL UNIONS AND NUCLEAR FUSIONS IN 
PLANTS. 
Tue forms of cell unions and nuclear fusions in plants fall 
into two groups: (1) 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 : — 
t. Protoplasmic connections between cells (plasmodesmen). 
2. Sexual cell unions and nuclear fusions. 
3. Asexual cell unions and nuclear fusions. 
1. 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 delicate strands of protoplasm so that contiguous pro- 
toplasts were not entirely separated from one another. This fact 
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