Fossil Plants and Living Fossils
1. Trends in Plant Evolution
Some words
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•
•
•
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Simple
Advanced
Primitive
Specialised
Ancient
The Plant Kingdom
• Algae [ several unrelated groups of plants]
• Mosses and Liverworts [ Bryophytes]
• Ferns, Horsetails, Club-Mosses and Quillworts
[Pteridophytes]
• Seed Ferns [all extinct]
• Seed Plants [Spermatophytes]:
– Gymnosperms
– Angiosperms:
• Dicotyledons
• Monocotyledons
Increasing complexity
• General trend is from simple structures to
more complex, specialised plants.
• Just because more-advanced plants groups
have evolved and exist it doesn’t mean that
the more-ancient plant groups have ceased to
exist (and, of course, the same applies in the
animal world).
Alga: Euglena
Alga: Pediastrum
Alga: Volvox
Alga: Spirogyra
Alga: Ulva
Alga: Laminaria
Alga: Fucus
Moss: Mnium
Liverwort: Marchantia
Club-Moss: Huperzia
Horsetail: Equisetum
Fern: Dryopteris
Seed Fern: Neuropteris
Gymnosperm: Cycas
Gymnosperm: Welwitschia
Gymnosperm: Pinus
Angiosperm, Dicotyledon: Magnolia
Angiosperm, Dicotyledon: Ranunculus
Angiosperm, Dicotyledon: Centaurea
Angiosperm, Monocotyledon: Lilium
Angiosperm, Monocotyledon: Dendrobium
Some more words
• The life-cycles of some plant groups include two
separate stages – the gametophyte (which
produces gametes, sex cells) and the sporophyte
(which produces spores).
• In mosses and liverworts, the plant you see is the
gametophyte and the sporophyte is the spore
capsule.
• In ferns et al, the plant you see is the sporophyte
and the gametophyte is a microscopic free-living
structure structure known as a prothallus.
Get out of the water
• Algae: Aquatic
• Mosses and Liverworts: Need very damp
conditions, but require dryness for Sporophyte
• Ferns and their Allies: Need dry conditions,
but require water for Gametophyte
• Seed Plants: Need dry conditions (aquatic
flowering plants have evolved back into the
water to take advantage of the habitat).
Enteromorpha in river
Moss gametophyte and sporophyte: Mnium
Fern sporophyte: Dryopteris
Fern gametophyte: Dryopteris
Aquatic Angiosperm: Victoria
Moving things around inside the plant
• The simplest plants have unstructured tissues,
where all the cells are the same and serve the
same function.
• As plants become larger there is an obvious need
to move material from one part of the plant body
to another: products of photosynthesis from
where they are made to where they are needed;
in vascular plants, water from the soil to the
extremities of the plant.
Vascular systems
• Earliest transport systems are in algae such as
Fucus.
• So-called ‘Vascular Plants’ (Pteridophytes and
Spermatophytes) have obvious veins running
through their tissues.
• Photosynthetic products move around in the
phloem.
• Water moves around in the xylem.
• The xylem of Pteridophytes and Gymnosperms is
composed of cells known as tracheids; that of
Angiosperms of open-ended vessels.
Fucus anatomy
Fern anatomy
Tracheids v Vessels
From spores to seeds
Algae/Bryophytes/Pteridophytes are known as nonflowering plants: they all reproduce by means of
spores.
Spermatophytes (as their names indicates) are
known as flowering plants and reproduce by means
of seeds (much more-complex structures).
Simple algae have no specialised reproductive
structures; more-advanced ones produce spores in
specialised structures (as, of course, do ‘higher’
plants).
Fern sporangia
Fern sporangia
Enteromorpha (entire plant body
produces spores)
Whole of blade of Laminaria covered
in sporangia
Brush-border of Laminaria sporangia
Receptacles in Fucus
Conceptacles in Fucus
Plurilocular sporangia in Pilayella
Pollination strategies
• Shape and colour of flowers in flowering plants is
related to what pollinates the flower – and, in the
case of animal pollinators, the plant has
sometimes evolved in parallel with their
pollinator.
• For example, the native New Zealand flora is
white because the islands separated from the
main continent early in insect evolution (nothing
that would see a yellow flower).
• Some orchids imitate their pollinator.
Wind-pollinated
Beetle-pollinated
Bat-pollinated
Bird-pollinated
Bee- and Wasp- Orchids
Taking advantage of change
Ideally, organisms are optimally adapted for the
habitat in which they are living. However, it is to
their advantage to be able to adapt to new or
changing environments: that, really, is what
evolution is all about.
The way to change comes through reproduction.
Strategies of reproduction
1. Asexual/Vegetative Reproduction: results in
offspring the same as the parents (and,
therefore, adapted to the same
environments).
2. Sexual Reproduction results in changes to the
genotype – genes from two parents, or
mutations during cell division – and the
offspring may, therefore, be (marginally)
better adapted to a (marginally) different
environment.
More on sexual reproduction
A plant may be:
1. Hermaphrodite (Monoecious) – produce both
male and female gametes and therefore to selffertilise.
2. Hermaphrodite – produce both male and
female gametes but have some mechanism to
prevent self-fertilisation.
3. Separate male and female plants (Dioecious)
so that cross-fertilisation is the only option.
The down-side
There is, of course, a down-side to this: the
more-specialised an organism becomes the
more restricted its choices may be. If it
reproduces sexually and is dependent on
another plant then, obviously, if another plant is
unavailable it is in trouble!
Vegetative reproduction: Fragaria runners
Vegetative reproduction:
Kalanchoe plantlets
Hermaphrodite: Anemone
Hermaphrodite: Primula Heterostyly
(Pin- and Thrum- flowers)
Heterostyly in Primula
Hermaphrodite: Arum (Male and female
parts of flower mature at different times)
Sexes separate (Dioecious): Taxus
Sexes separate (Dioecious): Ginkgo
250 million years separate these two
leaves of Ginkgo biloba (which may have
derived from seed ferns)