Although some ambiguity remains about the precise interrelationships of seed plants, comparisons with early, basal fossil seed plant taxa such as Hydrasperma and Pachytesta (see below) indicate that female gametophytes of extant cycads and Ginkgo likely retain a plesiomorphic pattern of development.
Basal seed plant female gametophytes cellularize through a highly distinctive process of alveolar formation [ILLUSTRATION FOR FIGURE 7A OMITTED].
Developmental patterns of extant basal seed plant female gametophytes (cycads, Ginkgo) and fossil evidence from some of the earliest known seed plants indicate that the plesiomorphic pattern of somatic female gametophyte ontogeny [TABULAR DATA FOR TABLE 1 OMITTED] among seed plants includes three critical phases: (1) free nuclear development; (2) centripetal cellularization (formation of alveoli); and (3) cellular growth and development accompanied by nutrient provisioning.
The ontogenies of all seed plant female gametophytes are determinate, nonmetameric (the sporophytes of seed plants have an indeterminate and metameric architecture), and as a consequence of being the sexual generation of the life cycle, involve the differentiation of gametes.
Publications on development of seed plant female gametophytes that include specific data about absolute size, rate of growth, and timing of fertilization are exceedingly rare (as was noted by deSloover 1963; our survey of the literature from the past century yielded only a few taxa for which requisite data are available).
gnemon is similar to that of plesiomorphic nonflowering seed plant female gametophytes and is probably greater than in Ephedra.
synchronous) among seed plant female gametophytes may well be linked to the preceding pattern (vacuolate vs.
As a consequence of their sexual and organismic status, seed plant gametophytes are focal points for selection and the consequent evolutionary modification of development (Friedman 1993).
gnemon continues to express the three phases of seed plant female gametophyte somatic development (free nuclear development, cellularization, cellular growth) common to more plesiomorphic seed plants.
multispermy) in Microcycas among extant seed plants is thus an
autapomorphy of Microcycas within extant seed plants (Fig.
to evolved at least six times in extant seed plants.
parallel development of diplospermy in other seed plants and other
Our work has shown that oleanane is lacking from a wide range of fossil plants," he notes, "but the chemical is found in Permian sediments containing extinct seed plants called gigantopterids.
That makes gigantopterids the oldest oleanane-producing seed plants on record--an indication that they were among the earliest relatives of flowering plants, concludes biologist David Winship Taylor of Indiana University Southeast, New Albany, a co-author of the study.