Entry of Pollen Tube into the Embryo Sac
The pollen tube enters the embryo sac through the filiform apparatus of ong of the synergids. Generally, one of the synergids degenerates before the entry of pollen tube and the tube invariably enters through such synergids. In Plumbago, where synergids are absent and the egg has the filiform apparatus, the tube enters directly into the egg. In several taxa, however, both the synergids remain healthy until the entry of pollen tube and the one that receives the pollen tube starts degenerating. It has been proposed that the degeneration of synergid and also of pollen tube cytoplasm is essential to prevent male gametes from a rejection or antigen-antibody type of reaction.
The choice of the male gamete involved in syngamy and triple fusion has long been a subject of speculation. Embryologists were curious to know which of the two male gametes participated in syngamy. Transmission electron microscopy and scanning electron microscopy have revealed that in certain plants the two male gametes are unequal in size and in the number of plastids and mitochonoria they contain. The plastid rich sperm seems to be preferentially involved in syngamy and the one with poor plastids fuses with the secondary nucleus. However recent studies indicate that heteromorphic sperms are not seen in other plants.