THE forms of cell unions and nuclear fusions in plants fall into two groups: (i) those which obviously have no sexual significance 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 problems 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 following heads :I. Protoplasmic connections between cells (plasmodesmen). 2. Sexual cell unions and nuclear fusions. 3. Asexual cell unions and nuclear fusions.