36-1Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Chapter 36: Plants

36-2Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Origin of plants• Plants colonised land c. 410 million years ago

– earliest plants were small– confined to wet margins of wetlands and rivers

• Plants arose from green algae (phylum Chlorophyta)

• Plants and chlorophytes both have– chlorophylls a and b– similar chloroplast structure– cellulose in cell walls– starch as storage material

(cont.)

36-3Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Origin of plants (cont.)

• 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.)

36-4Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Origin of plants (cont.)

• 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

36-5Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Adaptations to land

• Transition from water to land required structural adaptations to

– maintain water balance– extract water and nutrients from soil– transport water and nutrients around plant– support plant

36-6Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Features of plants

• Cuticle– waterproof layer of insoluble polymers and waxes that

covers above-ground parts of plants

• Sporopollenin– tough polymer of carotenoids protecting spores and

pollen

• Stomata– pores on surface of plant leaves and stems allowing gas

exchange

(cont.)

36-7Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

• Stems, roots and leaves– division of labour

36-8Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Reproduction • 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)

• Resulting zygote grows into diploid sporophyte generation, which produces haploid spores by meiosis

– zygote retained in gametophyte

36-9Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Heterospory

• 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

• Microspores in microsporangia– form male gametophytes producing sperm cells in antheridia

• Heterospory allowed seed plants to develop easily-transported pollen

36-10Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Non-vascular plants

• Phylum Hepatophyta (liverworts)

• Phylum Anthocerophyta (hornworts)

• Phylum Bryophyta (mosses)

• Vascular system absent• Lignin absent• Gametophyte generation dominant

36-11Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Gametophytes• Dominant generation in non-vascular plants• Gametophytes of non-vascular plants are leafy

(mosses) or thalloid (liverworts, hornworts)– attached to substrate by rhizoids

• Egg and sperm produced in archegonia and antheridia respectively

• Flagellated sperm swim to archegonia and fertilised egg

– sperm require water (rain, dew) for locomotion

36-12Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Sporophytes

• After fertilisation, sporophyte develops on gametophyte

• Sporangium (spore capsule) on stalk (seta)– derives nutrition from gametophyte

• 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

36-13Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 36.9: Life cycle of a moss (phylum Bryophyta)

36-14Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Spore-producing vascular plants

• 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

36-15Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Lycophytes• Lycopodium, Selaginella, Isoetes

– fossil lycophytes formed extensive forests, but modern species are small

• 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

36-16Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 36.19: Heterosporous life cycle of Selaginella (phylum Lycophyta)

36-17Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Filicophytes

• 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

36-18Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 36.23: Life cycle of homosporous fern

36-19Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Seed plants

• Gymnosperms• Phylum Coniferophyta (conifers)

• Phylum Cycadophyta (cycads)

• Phylum Ginkgophyta (ginkgos)

• Phylum Gnetophyta (gnetophytes)

• Angiosperms• Phylum Magnoliophyta (flowering plants)

36-20Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Ovules

• Megagametophyte develops within megasporangium

– megasporangium protected by cellular casings or integuments

– ovule is megagametophyte + megasporangium + integuments

• Megasporangium produces four megaspores by meiosis

– three degenerate, leaving one to grow into female gametophyte

36-21Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Seeds

• 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

36-22Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Pollen

• 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

36-23Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Secondary growth

• 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

• Cambium generates new phloem and xylem• Old vessels are filled with waste products

36-24Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Cycads

• 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

36-25Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Conifers

• Conifers are the most diverse group of non-flowering 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

36-26Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Magnoliophytes

Flowering plants (angiosperms) are divided into twogroups:• Monocotyledons (class Liliopsida)

– seeds have one cotyledon– flower parts in threes or multiples– leaves with parallel veins– vascular bundles in ground tissue in stem

• Dicotyledons (class Magnoliopsida)– seeds have one cotyledon– flower parts in fours or fives or multiples– leaves with netted veins– vascular bundles in a ring in stem

36-27Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Flower structure• 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)

• The superior ovaries of hypogynous flowers are attached to or above the receptacle

• The inferior ovaries of epigynous flowers are within the receptacle

36-28Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Origin of flowers• Classical theory

– flowers as modified leaves– stamens and carpels specialised leaf-like appendages

(sporophylls) being sporangia

• 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

36-29Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Evolution of flowers• 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.)

36-30Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Evolution of flowers (cont.)

• Flowers of magnolia family possess primitive characteristics

– bisexual, numerous stamens and carpels, pollinated by beetles

• Variety of other flower forms contemporaneous with early magnolias

• DNA evidence suggests that early flowers were small, with few parts in threes (trimerous flowers)

36-31Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Reproduction in flowering plants• Pollen lands on stigma and develops into the

microgametophyte– two non-flagellated sperm cells and one cell associated

with pollen tube growth

• In ovule, functional megaspore develops into megagametophyte

– embryo sac of eight nuclei in seven cells– one cell has two polar nuclei

• 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

36-32Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 36.40: Reproductive cycle of flowering plants (angiosperms)

36-33Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Pollinators• 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

36-34Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Pollination by deceit

• Some species of orchid mimic the shape and colouring of female insects

– a few species also mimic female pheromones

• Male insects attempting to mate with the flowers pick up pollen

• Pollen is brushed off onto stigma of the next flower visited

36-35Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fruits

• Fruits protect seeds and aid dispersal by fruit-eating animals

• Simple fruits– derived from ovary of a single flower with one or more

carpels (grape, apple, peas)

• Aggregate fruits – derived from clusters of carpels in a single flower

(raspberries, blackberries)

• Multiple fruits– derived from clusters of carpels from several flowers on

an inflorescence (pineapples)