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Ferns and Fern Allies • Tracheophytes (vascular plants) completed the conquest of the Earth begun by the primitive bryophytes • • • The evolution of the spore was the key to the bryophytes emergence onto land The evolution of vascular tissue and seeds let tracheophytes become fully terrestrial Over 279,000 species of tracheophytes (most of them flowering plants!) • Big split in plant evolution during the Early Devonian > Lycophytes - club moss etc. > Euphyllophytes - plants with true leaves • By the mid Devonian, euphyllophytes had split again into > Monilophytes - ferns and fern allies > Lignophytes - seed plants • • • • • True leaves (euphylls) formed as a web of tissue stretched between small terminal branches • • True leaves were an evolutionary response to this global drop in CO2 Thin flat blades of tissue were a more efficient way to capture an essential gas that was present in very low concentration • • Lycophyta, Sphenophyta, and Psilophyta are often lumped together as “fern allies” Ferns and fern allies are vascular plants that reproduce by means of spores rather than seeds • • • Ferns and fern allies are the most primitive living tracheophytes Tracheophytes have highly specialized roots, stems, and leaves Gametophytes and sporophytes usually have a symbiotic fungi (mycorrhizae) • Many similarities to bryophytes > Ferns and allies have free-swimming flagellated sperm, larger non-motile egg > Sperm must swim through water, so ferns and allies are limited to moist environment > Sporophyte develops directly from the gametophyte, no protection from desiccation • Differences between bryophytes and ferns > Sporophyte is the dominant stage in ferns > Ferns and fern allies are monoecious - antheridia and archegonia on same plant > Gametophytes are free-living plants, very small, only develop in moist areas Evolution of true leaves (euphylls) was a response to global environmental change Big global drop in carbon dioxide when vascular plants evolved and spread CO2 is a trace gas (!), much was taken up by early plants to make glucose through photosynthesis • Differences between bryophytes and ferns > Sporangia attached to sporophylls > Sporophylls organized into club-shaped strobili > Spread by rhizomes, modified underground stems that help spread it around • • Before ferns evolved, bryophytes were the dominant plants on Earth Tracheophytes quickly became the dominant flora > More highly evolved root-shoot system > Efficient vascular tissues > Larger size • • Ferns and fern allies formed planetary forest cover by the end of the Paleozoic, with trees 20-100 feet tall! Seed plants (primitive gymnosperms) evolved in the late Paleozoic, rapidly became the dominant plants • • Only ferns managed to compete with seed plants, and are still a significant group today Today most ferns and fern allies are small, inconspicuous plants Ferns Allies - Economic Importance • Most of our modern coal deposits were formed by horsetails, club mosses, and other trees during the Carboniferous (end of the Paleozoic) • • • • Phylum Lycophyta - club moss Phylum Sphenophyta - horsetails Phylum Psilophyta - whisk ferns Phylum Pterophyta – ferns Phylum Lycophyta - Club Moss • • • Phylum Lycophyta - club moss, qillworts, Selaginella 1,500 species, from Greek lycos = wolf - club moss, quillwort, Lycopodium, Selaginella Lycophytes are a sister group to ferns and other fern allies (monilophytes) • • Only five living genera, but for 400 my they were the dominant vegetation on Earth Now reduced to small plants on the forest floor • • Tropical species are mostly epiphytes (plants that grow on other plants) Temperate species grow in forest understory in small clusters • • Club mosses (Lycopodiales) are homosporous Selaginella and Isoetes are heterosporous • • • Heterosporous plants - gametes always come from two different gametophytes Homosporous can be same or different gametophytes Heterosporous plants are superior - increased variation for natural selection • • • • Club mosses, Selaginella, have sporophylls organized into strobili Cones falls to the ground when ripe, releases spores Gametophytes are independent, free living, look nothing like the parent plant Some species have autotrophic gametophytes, some have heterotrophic gametophytes Phylum Sphenophyta – Horsetails • • Phylum Sphenophyta - horsetails 25 species, from Greek sphen = wedge - horsetails, Equisetum • • • Closely related to ferns Equisetum is the only surviving genus of this Phylum Equisetum may be the oldest living plant genus on Earth • • Grows in waste places like roadside ditches or beside railroad tracks, a small and insignificant plant dreaming of former glory Evolved in the late Devonian, Equisetum was a dominant forest tree for several hundred million years Horsetails reached 30-60 ft. tall • • Leaves are little more than flattened stems Hollow stems are ribbed, jointed, whorl of leaves arise at each joint • • • • Stems feel very rough to the touch Pioneer women used them as scouring pads, hence the common name “scouring rush” Epidermal tissues are impregnated with tiny grains of silica (sand) - Why?? Well defended against herbivores • • Highly branched vegetative stalks (look like a horse’s tail) Unbranched reproductive stalks, tipped with a large strobilus bearing sporangia • Homosporous spores dispersed by elaters, develop into a tiny green gametophyte • Phylum Psilophyta - Whisk Ferns • • Phylum Psilophyta - whisk ferns 2 living genera, from Greek psilo = smooth - whisk ferns, Psilotum • • • Closely related to ferns Resemble the first vascular plants (convergent) Once large and widespread group of early land plants, now nearly extinct • • Only living vascular plants that lack true leaves or true roots (secondary loss) Found in tropical, subtropical habitats (southern US), common weed in greenhouses • • Small sporangia are bright yellow, form along the upper stems Gametophytes are tiny thread-like plants that lack chlorophyll, look like tiny fungi Phylum Pterophyta – Ferns • 12,000 species, from Greek pteridion = little wing - ferns, Pteris, Polypodium, Polytrichum • • • Evolved in the Devonian, share a common ancestor with whisk ferns Abundant and diverse, especially in the tropics (75% of species) Range in size from 1 cm to trees 24 meters tall, with 5 meter fronds • • • • Ferns are important in garden industry, florist trade Many herbal remedies derived from ferns Many fern species are edible (fiddleheads) Fern leaf blade is called a frond, leaflets are called pinnae • • Some ferns have separate fertile stalks which bear the sporangia Most ferns develop sporangia on the underside of their leaves • • Clusters of sporangia are called sori (sorus) Sorus often protected by an umbrella-like structure called an indusium (-ia) • • • Ferns are mostly homosporous, though some are heterosporous Spores are ejected by the sporangium, which acts like a miniature catapult Outer surface of the sporangia have thick and thin-walled cells • • Dry out at different rates, builds up enormous tension Top pulls back, reaches a critical point, snaps forward at incredible speeds, one of the most explosive acts in nature • • Diploid zygote develops into sporophyte Haploid spores formed by meiosis • • • Spores germinate into a tiny heart-shaped autotrophic gametophyte called a prothallus Archegonia and antheridia on upper surface - archegonia at the notch of the heart, antheridia near the rhizoids Sperm swims across to reach the egg • • Sporophyte grows out of the archegonia Early stage is called a fiddlehead, curled frond gradually unfurls and spreads out