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Chapter 28: Sexual reproduction Chapter 28: Sexual reproduction in the flowering plantin the flowering plant
Leaving Certificate BiologyLeaving Certificate BiologyHigher LevelHigher Level
Structure of the FlowerStructure of the Flower
Functions of the Flower PartsFunctions of the Flower Parts
• Receptacle:– Tissue from which all other parts originate
• Sepal:– Thick, green, leaf-like structures that protect the
developing flower when it is in bud form• Petals:– Large and brightly coloured in animal-pollinated
plants– Small and usually green in wind-pollinated plants
Functions of the Flower PartsFunctions of the Flower Parts• Stamen:– Male organ consisting of two parts:• Anther: pollen formation• Filament: supports the anther in a position where
pollen will be easily transferred
• Carpel:– Female organ consisting of three parts:• Stigma: pollen lands on stigma• Style: supports the stigma in a position where pollen
will have a good chance of landing• Ovary: where ovules develop
Male Gamete FormationMale Gamete Formation• The male gamete is the pollen grain• Pollen grain is a tough-walled single cell with
two nuclei:– Tube nucleus: burrows into the stigma and style
forming a tube (pollen tube)– Generative nucleus: will eventually fertilise the
egg
Pollen Grain DevelopmentPollen Grain Development
• Anther has 4 chambers called pollen sacs• Pollen sacs are where the millions of pollen grains develop
and mature• Each pollen sac has an outer fibrous layer (dermal tissue) that
protects the pollen sacs• Inside the protective layer is the tapetum – which nourishes
the developing pollen grains
Pollen Grain Development (cont.)Pollen Grain Development (cont.)
• On the innermost layer of the pollen sac is a layer of diploid cells (containing two sets of chromosomes) called microspore mother cells
• Microspore mother cells divide by meiosis (process of halving the number of chromosomes present in a cell) to produce four immature, haploid cells (containing single set of chromosomes)
Pollen Grain Pollen Grain DevelopmentDevelopment
Pollen Grain Development (cont.)Pollen Grain Development (cont.)• The immature, haploid pollen grains (microspores) then
mature over time and develop a tough outer wall called an exine (which is unique to the plant species) and a softer inner wall called the intine
• Mitosis of the haploid nucleus in each microspore also occurs during maturation – this produces a pollen grain with two haploid nuclei:– Tube nucleus: burrows into stigma and style– Generative nucleus: fertilises egg
Embryo Sac DevelopmentEmbryo Sac Development
• The ovary is located at the bottom of the flower with the style and stigma above it
• Within ovary are a number of ovules• Each ovule is composed of two outer wall
called integuments• Integuments have a small opening at the base
of the ovule, called the micropyle, that allows the pollen tube to enter and hence the fertilising nucleus to enter
Embryo Sac Development (cont.)Embryo Sac Development (cont.)
• The inner layer of each ovule has a layer called the nucellus – which nourishes the developing embryo sac
• Within each ovule are a number of diploid cells – one of which develops further to become the megaspore mother cell
• The megaspore mother cell divides by meiosis to produce 4 haploid cells
• Three of these haploid cell degenerate and one survives to become the embryo sac
Embryo Sac Development (cont.)Embryo Sac Development (cont.)
• The embryo sac (megaspore) enlarges and the haploid nucleus divides by mitosis to form 2 haploid nuclei
• The two haploid nuclei then divide again by mitosis to form 4 haploid nuclei within the one embryo sac
• Finally one more round of mitosis occurs to produce 8 haploid nuclei
Embryo Sac Development (cont.)Embryo Sac Development (cont.)
• The 8 haploid nuclei move to various areas of the embryo sac as shown (previous slide)
• Cell membranes and a thin cell wall form around 6 of the haploid nuclei and they split into groups of three and move to either end of the embryo sac
• The two remaining haploid nuclei remain free and are called polar nuclei
• Of the 6 haploid nuclei, 5 degenerate and one is left which is now called the egg cell
PollinationPollination• Pollination is the transfer of
pollen from anther to stigma of a flower of the same species
• There are two types:– Self-pollination: where a
flower allows pollen to fertilise the egg cell within the ovary of the same plant – disadvantageous to species as resulting seeds less likely to form healthy plant
– Cross-pollination: where a flower transfers pollen from anther to stigma of different plant of same species – more advantageous as greater variation is shown
Pollination MethodsPollination Methods
• Wind: pollen is produced in very large amounts by the flower and is usually small, light and smooth to allow easy transfer by wind, e.g., conifers and grasses
• Animal: pollen is produced in relatively small amounts grains are larger and stickier and they are usually transferred by insects (examples include dandelions, daisies, tulips, roses)
Wind Pollination Animal Pollination
Petals: small/absent, usu. green, no scent, no nectar
Petals: large, bright colour, scented, have food source (nectar)
Pollen: large amounts produced, light, small, dry, smooth
Pollen: small amounts, heavy, large, sticky, usu. Spiny
Anthers: large, outside flower, loosely attached to filament
Anthers: usu. small, inside flower, firmly attached to filament
Stigmas: large and feathery, outside flower
Stigmas: usu. small and sticky, inside flower
FertilisationFertilisation• Fertilisation is the union of the male and
female gametes to form a diploid zygote in sexual reproduction
Fertilisation (continued)Fertilisation (continued)• Once the pollen lands on stigma, pollen tube
forms by action of the tube nucleus• The generative nucleus enters the pollen tube
and divides by mitosis to form two haploid nuclei called sperm nuclei
• The sperm nuclei enter the embryo sac and ‘double fertilisation’ occurs:– One fertilises the egg – diploid (2n) zygote results– Other fuses with the two polar nuclei to form triploid
(3n) endosperm which functions as a food store• An adaptation of angiosperms to life on dry land
is pollen tube formation as no external water is required for fertilisation to occur
Seed FormationSeed Formation
• The ovule eventually becomes the seed:– Integuments become the testa– Zygote becomes the plant embryo– The embryo develops further into the radicle,
plumule, and cotyledon(s)– Triploid endosperm nucleus divides repeatedly by
mitosis to produce many cells that swell with food that comes from the nucellus
Endospermic Seeds versus Non-Endospermic Seeds versus Non-Endospermic SeedsEndospermic Seeds
• Endospermic seed:– The plant embryo increases in
size only absorbs some of theendosperm, e.g. Corn
• Non-Endospermic seed:– The plant embryo increases in size absorbing all of the
endosperm in the process e.g. Broad bean
Monocot versus Dicot SeedsMonocot versus Dicot Seeds• Monocot seeds: tend to be endospermic (e.g.
corn)– One cotyledon– When germinating the food is obtained mainly from
the endosperm– Tend to send up single shoot with no leaves (grasses)
• Dicot seeds: tend to be non-endospermic (e.g. Broad bean)– Two cotyledons– When germinating the food is obtained mainly from
the cotyledons– Send up shoots with leaves
Fruit FormationFruit Formation
• Fruits are formed from the ovary under the influence of auxins
• Fruits can also form from the receptacle of the flower (false fruits), e.g. apple
• Fruits protect seeds and attract animals to eat them so that seeds can be dispersed
Seed DispersalSeed Dispersal
• Dispersal is the transfer of the seeds away from the parent plant
• Advantages of dispersal are:– Avoid competition– Increases chances of surviving winter– Colonise new habitats– Increase the number of the species
Seed Dispersal (cont.)Seed Dispersal (cont.)
• Seeds can be dispersed in one of four ways:– Wind– Water– Animal– Self-dispersal
Seed Dispersal (cont.)Seed Dispersal (cont.)
• Wind dispersal:– Seeds are generally very light and usually have
some anatomical adaptation (hairs, wings) that enables them to be transported a long distance from parent plant, e.g. dandelions, sycamore
Seed Dispersal (cont.)Seed Dispersal (cont.)
• Water dispersal:– Seeds are usually enclosed within an air-filled fruit
that is capable of floating, e.g. water lillies, coconuts
Seed Dispersal (cont.)Seed Dispersal (cont.)
• Animal dispersal:– Seeds may be enclosed within a sticky fruit, e.g.
burdock, goosegrass– Seeds may be enclosed by a fleshy fruit, e.g.
strawberries, blackberries
Seed Dispersal (cont.)Seed Dispersal (cont.)
• Self-dispersal:– Seeds are enclosed within a pod that explodes
open when it becomes dry, e.g. pea pods
DormancyDormancy
• Dormancy is a resting period in which the seed undergoes no growth and has a very low metabolism
• Advantages of dormancy include:– Allows plant to avoid harsh conditions of winter– Gives embryo time to fully develop– Provides extra time for dispersal
Biotechnological IssuesBiotechnological Issues
• Seedless fruits• Larger fruits• Vegetable production• Ethene as a ripening agent• Dormancy of seeds in agriculture and
horticulture
Seedless Fruits & Larger FruitsSeedless Fruits & Larger Fruits• Parthenocarpy is the process of growing fruit
that do not have seeds• Parthenocarpy is carried out in two ways:– Breeding of plants in such a way as to produce
seedless fruit (pollination occurs but no fertilisation)– Use of auxins - auxins are sprayed onto plant and
stimulate fruit formation
• Parthenocarpy is linked to production of larger fruits as auxins causes fruits to become much bigger than normal during development
• Genetic engineering has also been used in producing larger fruit, e.g. tomatoes
Ethene as a Ripening AgentEthene as a Ripening Agent
• Ethene is a hydrocarbon (C2H4) gas that causes fruit to ripen (turn from green to characteristic colour)
GerminationGermination
• Germination is the regrowth of the embryo, following a period of dormancy, when the environmental conditions are suitable
• Factors necessary for germination:– Water– Oxygen– Suitable temperature
Digestion and Respiration in Digestion and Respiration in GerminationGermination
• Digestion of food substrates is required during germination as food stores in the form of oils and starch need to be mobilised and converted to usable forms – like fatty acids and glycerol and glucose
• Respiration is required to produce ATP as the embryo is growing and so anabolic reactions are occurring all the time (anabolic reactions require large amounts of ATP)
Stages of Seedling GrowthStages of Seedling Growth• There are two ways in which a seedling grows after
germination:– Cotyledons remain below the soil, e.g. broad bean– Cotyledons move above the soil, e.g. sunflower
Mandatory Experiment: Investigate Mandatory Experiment: Investigate Factors Affecting GerminationFactors Affecting Germination
• Set up 4 test tube as shown:
CONTROL NO H2O FRIDGE NO O2
Boiled waterOil layer
Cress seeds on cotton wool
Mandatory Experiment: To Show Mandatory Experiment: To Show Digestive Activity of a Germinating Seed Digestive Activity of a Germinating Seed • Set up apparatus as shown:
Cut seeds in halfSoak seeds for 2 days
Starch agar petri dishes
Control (boiled seeds) Test – live seeds