L. Hufford, THE MORPHOLOGY AND EVOLUTION OF MALE REPRODUCTIVE STRUCTURES OF GNETALES, International journal of plant sciences, 157(6), 1996, pp. 95-112
Male reproductive structures among extant Gnetales display a high leve
l of morphological correspondence. The two primary architectural eleme
nts in the male reproductive structures are cones and flowers. Cones c
onsist of an axis with bracts at each of the nodes and flowers that ar
e positioned in the axils of the bracts or on an annular mound located
above the bracts at each of the nodes. Flowers of the extant genera h
ave both a perianth and androecium. The perianth of all three genera h
as bracts in anterior and posterior positions, but Welwitschia has an
additional pair of lateral bracts. The androecium of all three extant
genera appears to be structurally homologous with a pair of highly syn
organized microsporophylls. Diversity is present among the extant gene
ra at both the cone and flower levels of organization. Some aspects of
diversification, such as the introduction of sterile ovules into male
reproductive structures, have affected both the cone and flower level
s of organization. Extant Gnetales appear to have radiated into most o
f the basic ''morphological spaces'' available to them for the placeme
nt of ovules. This may reflect enhancements for reproductive ecology,
especially the attraction of insect pollinators, that are available wh
en sterile ovules are present. Diversity among the extant Gnetales is
strongly biased in two aspects of development: phyllotaxy and synorgan
ization. All male reproductive structures appear to have the phyllotac
tic constraint of forming more than one appendage per node (or forming
a concrescent unit derived from the unification of structures that we
re more or less separate in an ancestor), and those of most species ha
ve a decussate arrangement of appendages along the reproductive axes.
Appendages at nearly all nodes of male reproductive structures are syn
organized, but within this bias they display various degrees of connat
ion. The complete unification of nodal appendages has created distinct
ive forms in Gnetum relative to Ephedra and Welwitschia. The arguments
presented for recognizing structural homologies among the features of
male reproductive structures of Gnetales should provide a stronger fr
amework for coding characters and their states for phylogenetic analys
es and for the discussion of morphological diversity among seed plants
.