Angiosperms and Gnetales (Ephedra, Gnetum, Welwitschia) represent the only
seed plants that regularly produce bisexual cones. Unfortunately, the ferti
lity and function of ovules formed on bisexual cones of Gnetales have remai
ned unclear. Some reports indicate that the ovules are sterile while others
indicate that they may develop into seeds. This study demonstrates three d
ifferent developmental patterns of ovules formed on bisexual cones of Gnetu
m gnemon. Type I ovules did not develop at all after pollination and repres
ented the majority of ovules on each cone. Type IT ovules enlarged slightly
after pollination due to the enlargement of nucellar tissue. Type LII ovul
es were typically found on the terminal whorl and developed into seed-like
structures. The enlargement was due to proliferation of megagametophyte tis
sue. Sectioned material revealed that megagametophytes show altered develop
ment compared to those found in functional female ovules. None of the ovule
s studied contained embryos, and thus all were sterile. Densitometry of 4 '
,6-diamidino-2-phenylindole (DAPI)- stained sections revealed that megagam
etophyte nuclei formed in the sterile ovules are unreduced (diploid) and th
us do not form viable female gametes.