Gymnosperms gymno = naked; sperma = seed
Lifecycle Evolution *** Significant advancements over seedless vascular plants *** • The seed contains an embryonic sporophyte • Protected and surrounded by nutrients for quick germination • Sporophyte grows into a large, usually woody plant • Gametophyte generation is tiny, dependent on the sporophyte and protected/enclosed by it • Female gametophyte remains in the ovule • Male gametophyte is the mature pollen grain • Pollen grain delivers the sperm via the pollen tube • Advantage: this frees gymnosperms from dependency on external water for transporting sperm to the egg • Now they can colonize drier environments!!! Yes!!
Leaves of Gymnosperms • Usually needlelike or leathery • “evergreen” • Coniferous, leaves kept year-round
Phylum Cycadophyta • Unique, ancient plants that are remnants of the lush Carboniferous period: “Age of Dinosaurs” • Often mistaken as palms, if strobili go unnoticed • Examples for research: Cycas and Zamia • Global distribution
Phylum Gnetophyta • This is a small and unique group (only 3 genera) • Ephedra is adapted to desert/grassland habitat • They are evolutionary important, which is thought to be most closely related to Angiosperms • Strobili in a flower-like cluster • Lacks archegonia • Performs double fertilization • Complex xylem vessels
Phylum Ginkgophyta • Unlike most, Ginkgo bilobais a deciduous gymnosperm • Fertilized ovules develop into flesh coated seeds (not fruit?)
Most Gymnosperms are conifers: bearing seeds in cones (strobili) • Phylum Coniferophyta – most numerous and widespread • 50 genera with about 550 living species • Earliest evolution some 300 m.y.a. with drought-resistant features • Tallest vascular plant: “California Redwood Sequoia” • Up to 117m tall and over 11m in trunk diameter • The female gametes: the ovules are NOT in ovaries, but are naked on the surface of cone scales or at the end of branches • Conifers are the most important plants in BC, economically, since most of the forestry industry is based on killing them • The four families (native to N.A.) of conifers found in BC are: • Pinaceae (pine family) • Cupressaceae (cypress family) • Taxodiaceae (bald-cypress family) • Taxaceae (yew family)
Lifecycle highlights • Examples from Pine species (highly representative of all) • 2-year lifecycle from early strobilus to seed development • Microsporangia are borne in Staminate cones (lower region) • Develop from modified leaf called Microsporophyll • Meiosis occurs to produce 4 haploid microspores (pollen grains) • Mitosis occurs to produce a 4-celled pollen grain = male gamete • Shed with wings via wind from the male cone in this condition • When it develops into a mature gametophyte only 2 cells remain • Once inside the female cone, one cell grows into a pollen tube • Fertilization occurs when the sperm nucleus unites with the egg cell • Result is a diploid (2n) zygote, further develops into an embryo within the seed
Lifecycle continues • Megasporangia are borne in “First Spring” Ovulate cones that are red/purple (usually occurs on higher branches) • Develop from a modified branch: seed-scale complex • Meiosis occurs to produce 4 haploid megaspores (one is functional) • Mitosis occurs to produce one mega-gametophyte (multiple eggs) • When non-motile sperm reach the archegonium, fertilization occurs • 6 months are needed to develop a mature seed • Result is a diploid (2n) zygote, further develops into an embryo within the seed coat (2n) but with haploid nutritive tissue
Grocery Store Botany • Gymnosperm medicinal products: • Ephedra from Gnetophyta • Lots from Pine spp • Gymnosperm edible products: • Dry Gin • Pine Nuts • Ginkgo White Nuts • Gymnosperm resins: • Varnishes • DNA extraction for Jurassic theme parks!
Flesh surrounding seed? • Yew and Juniper species have a fleshy layer surrounding the seeds, called an Aril (part of mega-gametophyte) • These species lack a normal cone. The flesh is haploid tissue • Not a fruit! But it is considered a “fruiting structure” • What selective advantage does this offer the conifers?
Evolution of 4-16 Nuclei in the Embryo Sac • The difference (is in the female cones) between Gymnosperms and Angiosperm tree-like organisms