Structure and Function of Female and Bisexual Organ Complexes in Gnetales

The female structures in Gnetales share the presence of an integument with a long, open micropylar tube that takes up the pollen grains via nucellar secretion presented at the micropylar apex. The micropylar tube is sheathed by one or two united pairs of outer envelopes that resemble the inflorescence bracts. Developmentally, Gnetales and angiosperms have in common a circular (inner) integument and its initiation at the time when the sporogenous cells become apparent, whereas the integument is two-lobed and its initiation precedes the differentiation of sporogenous cells in other gymnosperms. However, Gnetales differ from angiosperms in having an acropetal (and not basipetal) initiation sequence for the two inner envelopes, including the integument. The attachment of the two envelopes of the nucellus is at about the same level in most angiosperms, whereas the outer is much lower than the inner in Gnetales. The second or third envelope as counted from the nucellus (the carpel) is the receptive one in angiosperms, while it is always the innermost one (the integument) in Gnetales. In all three genera of the Gnetales bisexual organ complexes occur in various combinations but always with the female organs more apical than the male ones, in Ephedra at inflorescence (rarely flower) level, in Gnetum at partial inflorescence level, and in Welwitschia at flower level. The female organs in these bisexual complexes are often sterile but seem to be regularly functional by secreting a sweet pollination drop that serves as nectar for pollinating insects. In some species of Ephedra and Gnetum that lack female flowers, in the male inflorescences nectar may also be secreted by other parts of the inflorescence. Homologies of the female organ complexes with those of angiosperms are uncertain apart from the nucellus and the (inner) integument. The floral bracts (outer envelopes) may be apomorphic in Gnetales without direct correspondence in angiosperm flowers. The basipetal development of the bisexual complexes in Gnetum is compared with the basipetal development of the ovules in Bennettitales and a similar topology in conifers with several ovules on a seed scale.