Ontogeny of triovulate cones of Ephedra intermedia and origin of the outer envelope ofovules of Ephedraceae.

The origin of the two envelopes of ovules of extant species of Ephedraceae has been discussed previously in different ways based only on investigation of bimerous female cones. To generalize the characteristics of the two envelopes, ontogenetic and metamorphic patterns of the two envelopes of ovules of trimerous female cones of Ephedra intermedia Schrenk et C. A. Meyer were studied using scanning electron microscopy. The outer envelope of the ovules of triovulate female cones of Ephedra intermedia is initiated as two adaxial lateral protuberances. Thus, the outer envelope of extant Ephedraceae may have been derived from three foliar components of the second proximal whorl of the secondary reproductive shoot of the ancestral compound female cone. Two adaxial lateral foliar components likely gave rise to the outer envelope by fusion, while the third abaxial foliar component was reduced. These new findings suggest that the Gnetales derived their female cones from trimerous multiaxial female reproductive organs of their progenitors. Some evolutionary aspects of female cones of Ephedra are also discussed.