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“Describe the problems generated by life on land for the green plant lineage. How have these
problems been overcome?”
Introduction:
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Green plants are the dominant life form on the planet
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Make up almost 100% biomass
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Input energy into ecosystems
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Provide nutrients and niches for animals, fungi and bacteria
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Belong to single monophyletic group called the Embryophytes
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Include: flowering plants, conifers, ferns, clubmosses, mosses, hornworts and liverworts
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During transition phase, first land plants exposed to:
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Highly desiccating environment
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UV radiation
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Extreme temperature fluctuations
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Different conditions for reproduction and nutrient uptake.
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Aerial environment – altered gas exchange
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Increased gravity – increased mechanical support requirements
Advantages conferred by terrestrial life:
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Increased light availability
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Increased CO2 concentration in air than water
They have highly specialised morphological and physiological adaptations that allow them to
thrive on land.
Desiccation:
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Bryophytes (liverworts, hornworts & mosses) descend from earliest branching events in
phylogeny  evolved from Charophyceae algae
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Phylogenetic studies suggest that desiccation tolerance mechanisms have changed little
from the earliest land plants therefore dissociation resistance = primitive trait.
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Liverworts:
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Maximise surface area for absorption
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Hug moist ground (remain in contact with moist air near the ground)
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Suspend metabolism, growth and reproduction when they dry out but recover
quickly when water becomes available. This is possible because:
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Cellular structures stay in-tact so cellular integrity is regained upon
hydration.
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Photosynthetic machinery is similarly protected
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Transcription and translation to produce rehydrins helps to stabilise
membranes during rehydration.
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Hornworts & all vascular plants(sporophyte generation), mosses (sporophyte and gametophyte)
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Waterproof waxy cuticle covers epidermis of leaves, shoots and aerial organs.
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Made of cutin (a hydrophobic polymer)
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Forms barriers against water loss and prevent waterlogging when it rains.
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Prevents contamination by external water, dirt and microorganisms.
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Resistant to chemical and biological decay
All land plants (except liverworts) (in sporophyte generation)
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Stomata (pores in the epidermis) overcome gas impermeability
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Allow gas exchange (including CO2 and O2 entry for photosynthesis and
respiration respectively and by-product exit)
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Water vapour released through transpiration
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Stomatal aperture regulated by bordering pair of specialised guard cells
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Dehydrating conditions – ABA release causes stomatal closure – conserve
water
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Liverworts, cuticle doesn’t cover entire surface – gas exchange occurs on exposed
surfaces. More vulnerable to desiccation, but have H2O stores between cell walls for
back-up.
More efficient water transport system:
XYLEM
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Liberate from the constraints of small size and constant moisture.
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Specialised, unidirectional vascular transport system = XYLEM
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Transport water and soluble minerals passively from roots  bigger plants!
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Apoptosis  long continuous tubes formed from connected conducting cells.
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Xylem walls strengthened with lignin (polymer of polypropanoid monomers)
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Antibiotic
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Water-resistant
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Withstands negative pressures in xylem – compression resistance
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Lateral flexibility for bending and stretching – increase light absorption.
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Wide, empty cells more conductive than previous inter-cell method
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Enabled transport over long distances
ROOTS & PHLOEM
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Only bryophytes (first terrestrial plants) didn’t have roots – able to simply absorb nutrients
from surrounding water.
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Huge surface area to maximise water and nutrient absorption.
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Provide anchorage to substrate
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Increase stability
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Increase growth rate and potential plant size.
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Supply of photosynthate to roots needed for growth and metabolism bc dark conditions.
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Transport system = PHLOEM in vascular plants
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Living tissue
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Unlignified cells
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Transports sucrose to all parts of the plant (esp root and stem tips – growth)
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Hydrostatic pressure generates movement (achieved by phloem loading and
unloading)
Mycorrhizae:
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Endosymbiotic association between vascular roots of early land plants and a fungus.
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Host benefits:
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Increased mobilisation of phosphate from the soil (conc. 2-3 orders of magnitude
less than other mineral nutrients)
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Increased water and mineral nutrient absorption (bc of increases in surface area)
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Resistance to toxicity
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Plant growth hormone production by fungus
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Protection from pathogens
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Regulation of pH
Greatly enhanced plant root efficiency – they would not have been able to grow large
without efficient access to fungal-liberated phosphate.
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Therefore shaped evolution of land plants.
Protection against predators
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Link to mycorrhizae: other biotic interactions also shaped plant evolution
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Plants had to cross water/land interface already transiently occupied by animals.
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Had to deter predation without repelling animals involved in animal-mediated pollination and
seed dispersion - whilst remaining relatively immobile!
Physical defences:
Trichromes:
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Hairs – variable in presence and functionality among species
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Dense arrangement deters consumption by herbivores
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Distasteful and sharp fragment can penetrate the soft palate of the feeder’s mouth
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Causes intense and long-lasting irritation
Glandular trichromes: secrete pest- or pollinator-interactive chemicals
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Could impede animal movement
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Drosera – they secrete proteases which digest trapped animals – αα absorption
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Allows plant to survive in areas with low nitrogen availability (e.g. deserts)
Stinging hairs:
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When touched, fragile trichromes break and fragment
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Produces a sharp edge which can penetrate the skin.
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Histamine rapidly injected
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Immediate itching
5-hydroxytriptamne, Ach and a Na+ channel neurotoxin
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Prolonged burning sensation
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Reflect radiation to protect fragile tissues in hot and dry habitats
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Disrupt air flow across surface: reduces evaporation in windy conditions
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Formation of eddies in boundary layer increases CO2 diffusion through stomata.
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Prevent frost from touching the plant surface in freezing conditions
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Role in waterproofing therefore important in transition from land  air
Spines:
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Prevents grazing by larger herbivores
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Can form on almost all organs and surfaces
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Particularly well-developed in water-stressed habitats (e.g. deserts)
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Loss of photosynthetic area from grazing would be really damaging
Chemical defences:
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Deter pathogens and predators
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Synthesis of toxins
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Many aromatic so potential feeders can detect warning olfactory signals
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2 main classes: nitrogenous and non-nitrogenous
Non-nitrogenous
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Rotenone: produced in roots of a legume (Derris elliptica)
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inhibits cellular respiration – disrupts electron transport chain in mitochondria
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Particularly affects fish and insects (indigenous fishers and head-lice)
Nitrogenous: amino acid analogues, cyanogenic glucosides, alkaloids and proteins
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Amino acid analogues
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Many more amino acids in plant than are used as protein precursors – some toxic!
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If incorporated into animal proteins – become dysfunctional (esp. enzymes)
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E.g. legume seeds (high protein content)  protected by toxic amino acids
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(e.g. β-cyanoalanine in Vicia)
Causes convulsions and death
Proteins: Ricin (produced by Ricinus communis)
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Lethal in tiny amounts (1 microgram/kg body weight)
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Causes total failure of all protein synthesis
Cyanide
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Damage to plant tissues can cause cyanogenic glycosides (stored in vacuoles) to
combine with its degradative enzyme in the cytoplasm release of HCN gas (highly
toxic!)
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HCN acts as terminal respiration inhibitor – rapid death of predator
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Edible almonds are HCN-deficient mutants selected by humans and planted in
abundance. These seeds of these trees (‘sweet almonds’) are unprotected –
vulnerable to predation!
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Alkyloids
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Chemically characterised by nitrogen-containing rings
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Found in 20% of flowering plant families
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Liriodenine  produced by tulip tree  protect against parasitic mushrooms
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Strychnine: rat and human poison, provides arrow poison and curare (muscle
relaxant – used in anaesthetics)
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Atropa belladonna (deadly nightshade)  atropine
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Competitive agonist of ACh in muscarinic acetycholine receptors in
parasympathetic nervous system
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Increases heart rate, saliva production, pupils dilate
Animal hormones:
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Species of yew produces ecdysone (insect hormone that induces moulting)
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Quantities in plants much greater than animals
Reproduction: rapid modification needed for life on land!
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Plants are reproductively versatile
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Generates genotypic variation needed for evolutionary change.
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Can also reproduce clonally – prolong existence of single genotypes for 1,000’s of years!
Two generation life-cycle:
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Both generations are multicellular
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Can undergo sporic meiosis
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Diploid zygote divides by mitosis to generate multicellular diploid sporophyte
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Sporophyte divides by meiosis to produce haploid spores
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Thick-walled
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Desiccation resistant
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More likely to survive in hostile conditions
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Haploid spores divide by mitosis to produce haploid gametophyte
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Gametophyte divides by mitosis to produce gametes
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Gametes combine to produce zygote.
Nourishment of zygote and young sporophyte:
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In all embryophytes, eggs and embryos remain attached to gametophyte body
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In a structure called the archegonium
Jackets of sterile cells used as site for production of gametes (increased protection) and to
nourish the young sporophyte.
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Nutrients (e.g. sucrose) move through specialised transport cells from gametophyte to
sporophyte
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Transport cells have extensive invaginations of cell wall to increase PM area for transport
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Ensures large, well-nourished eggs
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Ensure large, well-nourished zygotes and embryos following fertilisation
Bryophytes (early land plants):
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New sporophytes grow and remain parasitic on the gametophyte
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Bc they lack functional chloroplasts
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In ferns, gymnosperms and angiosperms, sporophytes are independent of and dominant to
gametophytes.
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Vascular plants have green, autotrophic sporophytes which are now the dominant
phase of the life-cycle.
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The earliest land plants probably had two equal-sizes phases of the life-cycle.
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Selection against aerial sperm-producers would be very severe due to desiccation.
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Water-dependent fertilisation will be most reliable in contact with the humid ground (as in
many ferns) or even under the ground.
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Gametophytes of all living plants are ground-hugging, small and short-lived.
Clubmosses and ferns: spore producers!
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Large sporophyte produces haploid spores by meiosis in sporangia
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Spores are coated with sporopollenin
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Decay-resistant biological material
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Chemically stable
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Well-preserved in soils
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Spores released into the wind and germinate burrowed down in the soil.
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Divide by mitosis to produce small haploid gametophyte.
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Gametophyte differentiates to produce antheridia (sperm) and archegonia (ova)
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Symbiotic mycorrhizal relationship established – connect it to mother for nutrition
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These motile, ciliate sperm swim across moist soils to archegonia on the same or another
gametophyte – CROSS-FERTILISATION if densely crowded
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Fuse with ovule to produce embryo
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Embryo germinates – developing diploids push up through the soil to form sporophyte
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Sporophyte carries out independent photosynthesis
Angiosperms and gymnosperms:
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Spermatophytes (seed-producers) dominated earth ~250 mya.
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First = rapid proliferation of gymnosperms.
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During seed reproduction, fertilisation is truly internal.
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After fertilisation, tissues surrounding the embryo harden to produce a seed from the
whole ovule
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Seeds are therefore:
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Completely isolated from external environment
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Protected from desiccation
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Can enter dormancy for 10s-100s of years.
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Allowed plants to inhabit previously inhospitable areas (e.g deserts)
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Increased success rates of fertilised gametophytes
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Endosperm = nutrient store within the developing embryo
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Allowed seeds to germinate more rapidly – reach size at which they can
survive independently more quickly
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E.g. grow roots long enough to reach water table in arid environment.
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True internal fertilisation of angiosperms is controlled by the sporophyte through the
maternal tissue of the style – stigma and style can exercise ‘mate choice’ – discriminate
between pollen grains and tubes – eugenics!
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This enables angiosperms to increase genetic diversity by:
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Preventing self-fertilisation – can recognise ‘self’ and reject it
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Ensuring out-crossing
This is a form of sexual selection  maternal tissue selects the fittest pollen grains,
increasing the chances of successful fertilisation & increasing the genetic fitness of the
offspring.
Conclusion
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Evolution of land plants has resulted in increasing levels of complexity
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From Bryophytes to complex gymnosperms and angiosperms present today
They’ve evolved a diverse array of specific morphological, physiological and behavioural
adaptations to allow them to tolerate hostile conditions on land.
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Their colonisation of the land, and their subsequent diversification caused a series of
environmental changes, resulting in the generation of different habitats and the
development of entire terrestrial ecosystems.