Transcript 36-3 Origin of plants

Chapter 36: Plants
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
36-1
Origin of plants
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Plants colonised land c. 410 million years ago
– earliest plants were small
– confined to wet margins of wetlands and rivers
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Plants arose from green algae (phylum
Chlorophyta)
Plants and chlorophytes both have
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chlorophylls a and b
similar chloroplast structure
cellulose in cell walls
starch as storage material
(cont.)
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PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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Origin of plants (cont.)
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Closest relatives of plants are Charophytes (order
Charales) and Coleochaete
Charophytes have characters in common with
plants that are not found in other algae
– during cell division nucleus is not enclosed in nuclear
envelope
– cross-wall forms in phragmoplast, a structure containing
remnants of mitotic spindle fibres at right angle to new
cross-wall
(cont.)
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Origin of plants (cont.)
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Charophytes, Coleochaete and plants all possess
similar asymmetric, motile flagellated cells
Coleochaete and plants both possess reproductive
structures enclosed in protected cells
Charophytes and land plants both possess
glycolate oxidase and share similarities in 5 S
ribosomal RNA
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Adaptations to land
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Transition from water to land required structural
adaptations to
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maintain water balance
extract water and nutrients from soil
transport water and nutrients around plant
support plant
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Features of plants
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Cuticle
– waterproof layer of insoluble polymers and waxes that
covers above-ground parts of plants
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Sporopollenin
– tough polymer of carotenoids protecting spores and
pollen
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Stomata
– pores on surface of plant leaves and stems allowing gas
exchange
(cont.)
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Features of plants (cont.)
•
Vascular supply and lignin
– transport system allowing movement of water and
nutrients (xylem) and manufactured carbohydrates
(phloem)
– lignin in vessel walls prevents collapse
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Stems, roots and leaves
– division of labour
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Reproduction
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Plant life cycles involve alternating sexual and
asexual generations
Haploid gametophyte generation produces male
and female gametes by mitosis
– gametes protected in reproductive organs (male
antheridia, female archegonia)
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Resulting zygote grows into diploid sporophyte
generation, which produces haploid spores by
meiosis
– zygote retained in gametophyte
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Heterospory
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Some ferns and clubmosses and all seed plants
produce two types of spores in sporophyte
Megaspores in megasporangia
– form female gametophytes producing egg cells in archegonia
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Microspores in microsporangia
– form male gametophytes producing sperm cells in antheridia
•
Heterospory allowed seed plants to develop easilytransported pollen
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Non-vascular plants
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Phylum Hepatophyta (liverworts)
• Phylum Anthocerophyta (hornworts)
• Phylum Bryophyta (mosses)
• Vascular system absent
• Lignin absent
• Gametophyte generation dominant
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Gametophytes
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Dominant generation in non-vascular plants
Gametophytes of non-vascular plants are leafy
(mosses) or thalloid (liverworts, hornworts)
– attached to substrate by rhizoids
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Egg and sperm produced in archegonia and
antheridia respectively
• Flagellated sperm swim to archegonia and
fertilised egg
– sperm require water (rain, dew) for locomotion
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Sporophytes
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After fertilisation, sporophyte develops on
gametophyte
Sporangium (spore capsule) on stalk (seta)
– derives nutrition from gametophyte
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When spores are ready to be shed, spore capsule
open, exposing spores
– hygroscopic elaters in liverworts deform to flick spores
from capsule
– moss peristome changes shape, releasing spores
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Fig. 36.9: Life cycle of a moss (phylum Bryophyta)
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Spore-producing vascular plants
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Phylum Lycophyta (clubmosses, quillworts)
• Phylum Psilophyta (Psilotum, Tmesipteris)
• Phylum Sphenophyta (Equisetum)
• Phylum Filicophyta (ferns)
• Vascular system (xylem and phloem) present
• Lignin present in xylem
• Sporophyte generation dominant
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Lycophytes
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Lycopodium, Selaginella, Isoetes
– fossil lycophytes formed extensive forests, but modern
species are small
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Sporophyte generation dominant
Selaginella and Isoetes have heterosporous life
cycle
– mega- and microgametophytes develop within spore
– megagametophyte develops into archegonia with eggs
– microgametophyte develops into single antheridium with
flagellated sperm
– exposed when spore wall splits
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Fig. 36.19: Heterosporous life cycle of
Selaginella (phylum Lycophyta)
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Filicophytes
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Diverse plants with terrestrial, epiphytic and
aquatic species
Fern leaves (fronds) are characteristically divided
into pinnae or pinnules
Sporophyte generation dominant
– sporangia are clustered in sori and may be covered by a
protected membrane (indusium)
– small gametophyte bears archegonia and antheridia
– water is required for fertilisation
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Fig. 36.23: Life cycle of homosporous fern
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Seed plants
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Gymnosperms
• Phylum Coniferophyta (conifers)
• Phylum Cycadophyta (cycads)
• Phylum Ginkgophyta (ginkgos)
• Phylum Gnetophyta (gnetophytes)
• Angiosperms
• Phylum Magnoliophyta (flowering plants)
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Ovules
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Megagametophyte develops within
megasporangium
– megasporangium protected by cellular casings or
integuments
– ovule is megagametophyte + megasporangium +
integuments
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Megasporangium produces four megaspores by
meiosis
– three degenerate, leaving one to grow into female
gametophyte
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Seeds
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Plant embryos are surrounded by nutritive tissue
• Enclosed in a protective case (seed case or testa)
• Embryo remains dormant until it is shed from
parent plant and is dispersed to suitable habitat
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Pollen
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Microspores develop into microgametophytes,
which produce sperm
• Microgametophytes of seed plants develop as
pollen, which can be transported by wind, water or
animals
• Enclosing sperm in pollen means that seed plants
are not dependent on water as a medium allowing
sperm access to eggs
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Secondary growth
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Vascular cambium produces woody tissue
– large quantities of secondary xylem (wood) are produced
in the stem and roots adding girth
– smaller quantities of secondary phloem (bark) are added
to the outside of the plant
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Cambium generates new phloem and xylem
Old vessels are filled with waste products
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Cycads
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Ancient seed plants with a fossil record extending
back over 250 million years
Unbranched or sparsely branching trunk with
pinnate leaves
Specialised coralloid roots housing nitrogen-fixing
bacteria
Male and female plants produce cones bearing
sporangia and ovules respectively
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Conifers
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Conifers are the most diverse group of nonflowering seed plants
Branching trunk composed mostly of tracheids
(hence ‘softwood’)
Pollen and ovules are in separate cones
– pollen is dispersed by wind
– produces a pollen tube that conducts non-flagellated
sperm to archegonia
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Magnoliophytes
Flowering plants (angiosperms) are divided into two
groups:
• Monocotyledons (class Liliopsida)
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seeds have one cotyledon
flower parts in threes or multiples
leaves with parallel veins
vascular bundles in ground tissue in stem
Dicotyledons (class Magnoliopsida)
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seeds have one cotyledon
flower parts in fours or fives or multiples
leaves with netted veins
vascular bundles in a ring in stem
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Flower structure
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Flowers consist of four whorls of elements
– calyx of leaf-like sepals that protect flower
– corolla of petals
– stamens (each stamen consists of an anther and
filament)
– carpels (each carpel consists of a basal ovary containing
ovules, style and terminal stigma)
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The superior ovaries of hypogynous flowers are
attached to or above the receptacle
The inferior ovaries of epigynous flowers are within
the receptacle
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Origin of flowers
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Classical theory
– flowers as modified leaves
– stamens and carpels specialised leaf-like appendages
(sporophylls) being sporangia
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Genetic evidence supports sepals as modified
leaves
• Some petals may be modified leaves, others
sterile stamens
• Stamens may also represent a reduced branching
system with terminal microsporangia rather than
leaves
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Evolution of flowers
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Carpels are the most distinctive reproductive
structures of angiosperms
• Enclosure of ovules by carpels resulted in
modification of reproduction
• Carpels provide protection and formed specialised
fruits to assist in seed dispersal
• Pollen must grow a pollen tube through the stigma
to deliver sperm to the embryo sac
– creates possibility of selection by female choice
(cont.)
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Evolution of flowers (cont.)
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Flowers of magnolia family possess primitive
characteristics
– bisexual, numerous stamens and carpels, pollinated by
beetles
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Variety of other flower forms contemporaneous
with early magnolias
DNA evidence suggests that early flowers were
small, with few parts in threes (trimerous flowers)
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Reproduction in flowering plants
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Pollen lands on stigma and develops into the
microgametophyte
– two non-flagellated sperm cells and one cell associated
with pollen tube growth
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In ovule, functional megaspore develops into
megagametophyte
– embryo sac of eight nuclei in seven cells
– one cell has two polar nuclei
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Double fertilisation
– one sperm fuses with egg to form zygote
– other fuses with two polar nuclei to form triploid
endosperm nucleus, which then gives rise to nutritive
endosperm
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Fig. 36.40: Reproductive cycle of
flowering plants (angiosperms)
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Pollinators
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Plants produce flowers to attract pollinating
animals and maximise the chances of successful
pollination
– most animal-pollinated flowering plants produce showy
clusters of flowers (inflorescence)
– scent attracts pollinators (some night-flowering species
have strong scents)
– red coloration attracts birds, yellow and blue flowers
attract bees

some patterns reflect UV light
– tubular shape of some flowers ensures that animals have
to brush against anthers to get nectar
– nectar provides a high-energy lure
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Pollination by deceit
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Some species of orchid mimic the shape and
colouring of female insects
– a few species also mimic female pheromones
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Male insects attempting to mate with the flowers
pick up pollen
• Pollen is brushed off onto stigma of the next flower
visited
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Fruits
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Fruits protect seeds and aid dispersal by fruiteating animals
Simple fruits
– derived from ovary of a single flower with one or more
carpels (grape, apple, peas)
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Aggregate fruits
– derived from clusters of carpels in a single flower
(raspberries, blackberries)
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Multiple fruits
– derived from clusters of carpels from several flowers on
an inflorescence (pineapples)
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