Download Ferns and Fern Allies

Ferns and Fern Allies
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Tracheophytes (vascular plants) completed the conquest of the Earth begun by the
primitive bryophytes
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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!)
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Big split in plant evolution during the Early Devonian
> Lycophytes - club moss etc.
> Euphyllophytes - plants with true leaves
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By the mid Devonian, euphyllophytes had split again into
> Monilophytes - ferns and fern allies
> Lignophytes - seed plants
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True leaves (euphylls) formed as a web of tissue stretched between small terminal branches
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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
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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
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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)
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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
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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
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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
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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
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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
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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
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Most of our modern coal deposits were formed by horsetails, club mosses, and other trees
during the Carboniferous (end of the Paleozoic)
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Phylum Lycophyta - club moss
Phylum Sphenophyta - horsetails
Phylum Psilophyta - whisk ferns
Phylum Pterophyta – ferns
Phylum Lycophyta - Club Moss
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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)
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Only five living genera, but for 400 my they were the dominant vegetation on Earth
Now reduced to small plants on the forest floor
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Tropical species are mostly epiphytes (plants that grow on other plants)
Temperate species grow in forest understory in small clusters
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Club mosses (Lycopodiales) are homosporous
Selaginella and Isoetes are heterosporous
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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
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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
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Phylum Sphenophyta - horsetails
25 species, from Greek sphen = wedge - horsetails, Equisetum
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Closely related to ferns
Equisetum is the only surviving genus of this Phylum
Equisetum may be the oldest living plant genus on Earth
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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
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Leaves are little more than flattened stems
Hollow stems are ribbed, jointed, whorl of leaves arise at each joint
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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
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Highly branched vegetative stalks (look like a horse’s tail)
Unbranched reproductive stalks, tipped with a large strobilus bearing sporangia
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Homosporous spores dispersed by elaters, develop into a tiny green gametophyte
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Phylum Psilophyta - Whisk Ferns
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Phylum Psilophyta - whisk ferns
2 living genera, from Greek psilo = smooth - whisk ferns, Psilotum
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Closely related to ferns
Resemble the first vascular plants (convergent)
Once large and widespread group of early land plants, now nearly extinct
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Only living vascular plants that lack true leaves or true roots (secondary loss)
Found in tropical, subtropical habitats (southern US), common weed in greenhouses
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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
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12,000 species, from Greek pteridion = little wing - ferns, Pteris, Polypodium, Polytrichum
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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
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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
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Some ferns have separate fertile stalks which bear the sporangia
Most ferns develop sporangia on the underside of their leaves
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Clusters of sporangia are called sori (sorus)
Sorus often protected by an umbrella-like structure called an indusium (-ia)
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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
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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
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Diploid zygote develops into sporophyte
Haploid spores formed by meiosis
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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
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Sporophyte grows out of the archegonia
Early stage is called a fiddlehead, curled frond gradually unfurls and spreads out