Chap 30: Plant Diversity II
The Evolution of Seed Plants
Overview of Seed Plant Evolution
So in this section you are answering the essay question: What reproductive
adaptations did seed plants (gymnosperms and angiosperms) make to be
successful terrestrial organisms?
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Reduction of the gametophyte generation
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The gametophyte generation was getting pretty small with the ferns
(seedless) but it is even smaller and more “insignificant” with the
seed plants.
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It is microscopic and cannot be seen with the naked eye.
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In seed plants the female gametophyte develops from a spore that is
retained within the sporangia. See Fig 30.1c.
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Advantage: no environmental stress is experienced.
•The gametophyte and embryo are protected from drought and
UV radiation.
•Nutrients can also be gotten from the parent.
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Why has the gametophyte not disappeared from the life cycle?
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screening of new mutations
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gametophyte helps to nourish the sporophyte embryo
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Figure 30.1 Three variations on gametophyte/sporophyte relationships
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Seeds became an important means of dispersing offspring
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A spore is a single cell whereas a seed an embryo plus food supply.
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The seed develops within the female gametophyte which is retained
on the parent plant (sporophyte)
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Within the ovule develops one or more egg cells which when
fertilized develops into a zygote and remains protected.
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The seed’s protected coat develops from the integument of the ovule.
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Figure 30.2 From ovule to seed
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Figure 30.3 Winged seed of a White Pine (Pinus strobus)
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Pollen eliminated the liquid-water requirement for fertilization
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So this is an important point since one of the main aspects of
coming onto land was keeping a moist environment for the
sperm to find the egg. Now we can get rid of this
adaptation.
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Pollen grains develop from the microspores which produce
the male gametophyte.
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Pollen is protected by the sporopollenin coats.
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When pollen nears the ovule it produces a pollen tube
through which sperm travel to reach the female
gametophyte.
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Most of these sperm lack flagella
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Airborne pollen not swimming of sperm
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The two clades of seed plants are gymnosperms and angiosperms
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Different ancestors of gymno- and angiosperms but both came from
the progymnosperms.
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Two key reproductive adaptations of seed plants:
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seeds
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pollen
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Gymnosperms
The Mesozoic era was the age of gymnosperms
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Gymnosperms replaced the club mosses, horsetails and ferns as arid
conditions increased on Earth some 250 million years ago.
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Gymnosperms were a main food for dinosaurs
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Gymnosperms persisted when dinosaurs went extinct.
Four Phyla
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Ginkgo (Ginkgo biloba)
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Cycads
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Gnetophyta
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Conifers
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Figure 30.5c Phylum Ginkgophyta: Ginkgo biloba
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Figure 30.6 Phylum Cycadophyta: cycads
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Figure 30.7a Phylum Gnetophyta: Welwitschia
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Figure 30.8a Phylum Coniferophyta: Douglas fir
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Figure 30.9 The life cycle of a pine (Layer 3)
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Angiosperms
Angiosperm Clades
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Phylum: anthophyta
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Not simply divided into monocots and dicots anymore
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Monocots: lilies, orchids, palms, grasses
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venation in leaves runs parallel
Eudicots
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includes the majority of dicots
•Netlike venation
•Roses, peas, sunflowers, oaks and maples
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Other lineages
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What contributed to the success of the angiosperms?
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Xylem tissue with tracheids for mechanical support and transport
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Fiber cells also for support
• Vessel elements which are more efficient at transporting water
than tracheids.
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Reproductive adaptations of flowers and fruits.
The flower is the defining reproductive adaptation of angiosperms
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Know fig 30.13
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Fruits help disperse the seeds of angiosperms
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What you are eating is an ovary.
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The ovule develops around the seed
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Pericarp: wall of the ovary/fruit
Fruit Classification
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Simple fruit: derived from one ovary; cherries, soybean pod and
pea pod
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Aggregate fruit: single flower with several carpels, each carpel
having an ovary and therefore producing lots of
“little fruits” and since these carpels are close
together they form an aggregate; blackberries
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Multiple fruit: the flowers of this fruit are so close together that
there fertilized ovaries fuse as they develop and
become one fruit; pineapples.
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Figure 30.17 The life cycle of an angiosperm
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The radiation of angiosperms marks the transition from the Mesozoic era to the
Cenozoic era
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Oldest fossils are about 130 million years ago
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They became the dominant plants within 65 million years and are still
the dominant plant today.
Angiosperms and animals have shaped one another’s evolution
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Coevolution: the influence two different species have on each other’s
evolution through their interactions and thus affecting the selected
adaptations of each organism.
• insects were favored to evolve with those plants that kept their
reproductive parts off of the ground.
• animals that carried a plant’s pollen or seeds were selected for by
obtaining food on the one hand and helping the plant to reproduce on
the other.
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Plants and Human Welfare
So what is the value of plants to humans?
Agriculture is based almost entirely on angiosperms
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Flowering plants provide most of our food. Vegetables and fruits are
both angiosperms
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Selective breeding of plants became a way to improve crop
productivity and quality.
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Table 30.2: Angiosperms are also valuable as medicines
Plant diversity is a nonrenewable resource
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Table 30.2 A Sampling of Medicines Derived from Plants
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Figure 30.12 Xylem cells in angiosperms
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Figure 30.13a The structure of a flower
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Figure 30.4 Hypothetical phylogeny of the seed plants
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