Question

Double fertilization followed by the formation of endosperm is unique to angiosperms. How does double fertilization in the Gnetophyta, Ephedra and Gnetum, differ from that in angiosperms?

Answer 1

Subdivision: Gnetophyta

This subdivision consists of 3 living genera and approximately 70 species, some quite unusual and still very apparent today:

Gnetum-grow in the tropics, 30 species, grows usually as trees or as vines with large leathery leaves:

Ephedra-the 35 species are mostly shrubby, with small scale like leaves and jointed stems. They live mostly in arid or desert areas. It continues to be used as a medicinal tea. Jump to the ephedra page for more information.

Welwitschia-grows in the deserts of SW Africa, each leaf has a meristem to repair damaged tissue, has male and female cones. GO on to the the weltwitschia page for more information about this highly unusual plant...

How double fertilization in Gnetophyta is different from double fertilization in angiosperms?

• Gnetophyta's unique fertilization feature is that a tube grows from the eggs to unite with the pollen tubes in order for fertilization to take place between the gametophytes. The sperm themselves are not motile, like we see in the cycads and gingko.
• This is the only type of gymnosperm that undergoes double fertilization, however no endosperm forms and the 2nd egg fertilized disintegrates. Also like the angiosperms, some of the species produce nectar and are visited by insects.
• It is thought that it is this group may have evolved the angiosperms- but certainly not any of the extant species .. most likely an early progenitor of both this group and the angiosperms.