Plant Diversity II
Level 1 Biological Diversity
Jim Provan
Campbell: Chapter 30
Reproductive adaptations of seed plants
Three life cycle modifications led to the success of
terrestrial plants:
Reduction of the gametophyte: retained in the moist
reproductive tissue of the sporphyte
Origin of the seed:
—
—
Zygotes developed into embryos packaged with a food supply
within a protected seed coat
Seeds replaced spores as the main means of dispersal
Evolution of pollen: plants were no longer tied to water for
fertilisation
Reduction of gametophytes in seed plants
Gametophyte (n)
Sporophyte (2n)
Sporophyte (2n)
Sporophyte (2n)
Gametophyte (n)
Gametophyte
(n)
Sporophyte dependent on
gametophyte (e.g. bryophytes)
Large sporophyte and small
independent gametophyte
(e.g. ferns)
Reduced gametophyte
dependent on sporophyte
(seed plants)
In seed plants, the seed replaced the spore
as the main means of dispersing offspring
Relatively harsh terrestrial environment:
Bryophytes and seedless vascular plants release spores
Seeds are more hardy because of their multicellular nature
Seed is a sporophyte embryo and a food supply
surrounded by a protective coat
All seed plants are heterosporous
Development of seed associated with megasporangia:
Seed plant megasporangia are fleshy structure called nucelli
Additional tissues (integuments) surround megasporangium
Resulting structure is called an ovule
Female gametophyte develops in wall of megaspore, is
fertilised (embryo) and resulting ovule develops into a seed
From ovule to seed
Integuments (2n)
Nucellus
(megasporangium)
(2n)
Spore case (n)
Megaspore (n)
Pollen tube (n)
Micropyle
Female
gametophyte (n)
Egg
nucleus
(n)
Discharged sperm
nucleus (n)
Seed coat (2n)
derived from
integuments
Food supply
(derived from
female
gametophyte
tissue
Embryo (2n)
new sporophyte
Pollen became the vehicle for sperm
cells in seed plants
Microspores develop into pollen grains which mature
to form the male gametophytes of seed plants:
Pollen grains coated with a resistant polymer, sporopollenin
Can be carried away by wind or animals (e.g. bees)
following release from microsporangia
A pollen grain near an ovule will extend a tube and
discharge sperm cells into the female gametophyte
within the ovule:
In some gymnosperms, sperm are flagellated (ancestral)
Other gymnosperms (including conifers) and angiosperms do
not have flagellated sperm cells
Gymnosperms
Descended from Devonian
progymnosperms:
Seedless
Seeds evolved late Devonian
Climatic changes during
Permian led to lycopods,
horsetails and ferns being
replaced by conifers and
cycads
Lack enclosed chambers
(ovaries) in which seeds
develop
Four divisions of extant gymnosperms
Cycads
Gnetophytes
Gingko
Conifers
Conifers are the largest division of
gymnosperms
Mostly evergreens e.g.
pines, firs, spruces, larches,
yews, cypresses etc.
Include the tallest, largest
and oldest living organisms
Needle-shaped leaves
adapted to dry conditions:
Thick cuticle covers leaf
Stomata in pits, reducing
water loss
Megaphylls cf. other leaves
The life cycle of a pine
Angiosperms
Angiosperms (flowering plants)
Flowering plants are the
most widespread and
diverse (250,000 species)
Only one division
(Anthophyta), with two
classes:
Monocotyledons
Dicotyledons
Less dependent on wind
pollination - use insects
and animals
Evolution of vascular tissue in
angiosperms
Conifers have waterconducting cells called
tracheids
Angiosperms have vessel
elements:
More specialised for transport
Less specialised for support
Xylem reinforced by fibres:
Specialised for support - thick
lignified wall
Evolved in conifers (conifers
lack vessel elements)
The flower is the defining reproductive
adaptation of angiosperms
Sepals: sterile, enclose
bud
Petals: sterile, attract
pollinators
Stamen: produces pollen
Carpel: evolved from seedbearing leaf that became
rolled into a tube
Stigma: sticky structure
that receives pollen
Ovary: protects ovules,
which develop into seeds
after fertilisation
Fruits help disperse the seeds of
angiosperms
Fruits are ripened ovaries
that protect dormant seed
and aids in its dispersal
Modifications of fruits that
aid dispersal include:
Seeds within fruits that are
shaped like kites or
propellors (e.g. maple)
Burr-like fruit that cling to
animal fur
Edible fruit - tough seeds
pass through digestive tract
Life cycle of an angiosperm
Anther
MITOSIS
Ovule
Germinating
seed
Stigma
Pollen
tube
Ovary
Food supply
Seed
Pollen
tube
MITOSIS
Style
Pollen
tube
Angiosperms and animals shaped
one another’s evolution
Coevolution: reciprocal
evolutionary responses
among two or more
interacting species
Coevolution led to diversity
of flowers:
Flower-specific pollinators
Usually adapted for types of
pollinators
Attraction of ripening fruits:
Soft, fragrant and sugary
Attractive change of colour
Plants transformed the atmosphere
and the climate
Colonisation of land by plants
Diversification of vascular plants
25
20
15
10
5
Plants decreased
atmospheric carbon
dioxide, resulting in
global cooling
Cooler environment
made terrestrial life
more habitable for other
organisms