Phylogenetic relationships within and between Gymnosperms and Angiosperms have been the subject of intensive research over the past few years; some of these results are summarized HERE. In examining these trees, note where the results depict relationships between the flowering plants and the different gymnosperm groups.
|Gymnosperms (Pinophyta)||Angiosperms (Magnoliophyta)|
|see also this page re the structure of wood|
|Phloem||Phloem mother cells develop into sieve-cells only.||Phloem mother cells develop into sieve-cells and companion-cells.|
|Microsporangia||Arranged in unisexual strobili.||Arranged in bisexual (sometimes unisexual) strobili (flowers).|
|Megasporangia||In unenclosed ovules, on megasporophylls, on compressed short shoot-bract complexes, or borne terminally on leafy shoots.||In ovules enclosed by tissue of the parent sporophyte (ovary) at the apex of bisexual (sometimes unisexual) strobili (flowers).|
|Pollination||By wind, directly to micropyle of ovule.||By wind, water, and especially by animal vectors; intercepted by stigma where pollen germination occurs. Compatible pollen tubes grow through sporophytic tissue to micropyle of ovule to deliver sperm nuclei to megagametophyte.|
|Gametophytes||Megagametophyte massive, provides nourishment for embryo (pre-fertilization provisioning); microgametophyte three to tens of cells.||Megagametophyte highly reduced, 4- to 16-nucleate, derived from one, two, or four megaspores; microgametophyte of three cells.|
|Fertilization||Double fertilization absent (except in Ephedra, Gnetum).||Double fertilization
present (leading to formation of endosperm plus the embryo that it
nourishes); provisioning of the embryo thus takes place post-fertilization.
Angiosperm Life Cycle animation (courtesy Heather Kroening & Bio-DiTRL)
|Seeds||Typically unenclosed, in simple or compound cones; predominantly wind-dispersed, some animal dispersal (e.g. Taxus) .||Enclosed in fruit at least until mature; dispersed via water, wind, and animals (externally, internally).|
|Species||ca. 600.||> 200,000.|
|Distribution, ecology||Predominantly temperate zone trees, shrubs (only Cycadales exclusively tropical or subtropical).||Present in virtually all biomes, and representing virtually all life forms. Size 2 x 10-3 (Lemna) to 102 m (Eucalyptus), duration 10-1 to 103 y. Photosynthetic efficiency, especially under conditions of high temperatures and limited water availability, enhanced by variants (C4, CAM) of the C3 carbon fixation pathway.|
Compare the ovules of Ginkgo (at right; scale in cm), as seen in the Gymnosperm lab with those of Lilium (below, in longitudinal section; scale as indicated). The Ginkgo ovules contain a massive megagametophyte that is approximately 1 cm in diameter, and in which we were able to observe archegonia with egg cells about 0.5 mm in diameter.
In the Lilium ovules, the megaspore (left, below) develops into a few-celled megagametophyte (center, below) in which three nuclei adjacent the micropyle are analogous to an archegonium (one of them functions as an egg nucleus).
The progressive reduction of the gametophyte that is seen in the seed plants is an example of neoteny or, more generally, of heterochrony. In this case (neoteny), reproductive maturity is achieved earlier and earlier in development. Compared to the perennial, woody habit, the herbaceous annual habit of many angiosperms can also be seen as an example of neoteny.
For more info on reproduction in flowering plants, including links to still more sites, click HERE.
Ginkgo ovules - photo: E. Harris. Lilium philadelphicum - photo: M. Ferguson. Both © 1999, 2000 Royal Ontario Museum.
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© 2008 Department of Ecology & Evolutionary Biology and 2000-2006 Botany Department, University of Toronto.