How does Welwitschia fertilization differ from that of other gymnosperms?

Welwitschia fertilization differs significantly from that of other gymnosperms due to its unique reproductive development. Key distinctions include:12

• Absence of Archegonia: Unlike most other gymnosperms, which form archegonia within the female gametophyte to house the egg, Welwitschia does not produce archegonia at all.12
• Multinucleate Gametophyte: Following meiosis in the ovule, the resulting nuclei undergo repeated mitoses without forming cell walls, creating a multinucleate female gametophyte.2
• Unique Fertilization Mechanism: Instead of a pollen tube growing down to a stationary egg, some cells of the multinucleate female gametophyte grow upward into the nucellus. After pollination, pollen tubes grow downward from the male gametophyte and fuse with these upward-growing female tubes.2
• Fertilization Bulbs: Fusion between the male gamete and a nucleus from the female gametophyte occurs within specialized structures called "fertilization bulbs," after which the resulting zygote grows downward into the cellular portion of the female gametophyte to develop into an embryo.2

Additionally, Welwitschia ovules possess two integuments, a feature more commonly associated with angiosperms than other gymnosperms.1