How does a pollen tube help fertilization occur in gnetophytes?

In gnetophytes, the pollen tube is essential for conveying male gametes to the female gametophyte to achieve fertilization. After pollination, the pollen grain germinates on the nucellus, and the resulting pollen tube grows downward through the tissue.12

The specific mechanism of fertilization varies among the genera:

• In Ephedra: The pollen tube enters the egg cell of the archegonium, where it discharges a male gamete to fuse with the egg nucleus.2
• In Gnetum and Welwitschia: These genera lack archegonia. Instead, the pollen tubes grow downward through the nucellus and fuse with upwardly growing multinucleate tubes originating from the female gametophyte. This fusion creates "fertilization bulbs," within which a male gamete fuses with a nucleus from the female gametophyte to form a zygote.12

In all cases, the pollen tube serves as the critical conduit that enables the non-motile sperm to reach the female gamete, facilitating the transition from the gametophyte phase to the sporophyte phase.12