AlCh12b

12.3 Reproduction in Flowering Plants: 

12.3 Reproduction in Flowering Plants

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Vegetative structures: stems, roots and leaves Reproductive structures: flowers Vegetative Propagation: reproduction of vegetative structures

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Sexual reproduction: introduce variation through meiosis by 1. Independent assortment chromosomes during metaphase 2. Recombination of genes by crossing over between homologous chromosomes at prophase I

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12.3.1 Floral Structure Both self-fertilization and cross-fertilization introduce variation. External agents, e.g. insects , help in transfer of genetic material when they assist in pollination of flowers. This, however, exposes the vulnerable gametes to desiccation. Spermatophytes thus have evolved their male gamete within a spore, the microspore or pollen grain.

Structures of a typical flower:: 

Structures of a typical flower: Angiosperms reproduce sexually by producing flowers

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stomium liberated pollen grain

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The flower is the organ of sexual reproduction in flowering plants Flowers are usually hermaphrodite or bisexual but sometimes unisexual sepals: outermost whorl; form the calyx; green in colour; Function - 1 Protect the flower bud 2 Carry out photosynthesis

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Petals: form the corolla; often brightly coloured & attractive; with insect guides which guide insects to the base of the petals for nectar secreted by the nectary; In some flowers, the calyx & corolla are undistinguishable and are collectively called the perianth

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Stamens: male reproductive parts or androecium; Each stamen consists of an anther and a long filament; Each anther contains four pollen sacs with pollen grains inside; The pollen grains contain the male gametes

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Carpels: female reproductive parts or gynoecium (or pistil); Each carpel consists of an ovary which contains ovules with ova inside; At the top of the ovary is the style, with a stigma at the tip to receive pollen grains for fertilization

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Actinophmorphic (regular): flowers with petals & sepals of similar size and shape exhibiting radial symmetry Zygomorphic (irregular): flowers with unequal sepals & petals of different shapes and arranged in bilateral symmetry

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12.3.2 Pollination

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12.3.2 Pollination - meiosis occurs inside the pollen sacs of anther; When mature, pollen sacs split open expose their pollen grains - female gametes (egg nuclei) are inside ovules funicle: ovule stalk integuments: protective layer micropyle: a small opening through which pollen tube grows into ovule

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Development of the pollen grain(for reference): pollen mother cells (2n)  meiosis  tetrad (4) haploid (n) cells  microspores (pollen grains)  mitosis  generative nucleus & tube nucleus  mitosis  2 male nuclei & tube nucleus

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Development of the ovule (for reference): nucellus  megaspore mother cell (2n)  meiosis  4 megaspores (n)  3 degenerated, 1  embryo sac  mitosis 3 times  8 nuclei (3 antipodal cells, 2 polar nuclei, 2 synergids with 1 egg cell)

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Pollination: the transfer of pollen grains from anthers to the stigmas; external agents are needed – (1) by insect (2) by wind

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Comparison of anemophilous (wind-pollinated) and entomorphilous (insect-pollinated) flowers

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Comparison of anemophilous (wind-pollinated) and entomorphilous (insect-pollinated) flowers

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Comparison of anemophilous (wind-pollinated) and entomorphilous (insect-pollinated) flowers

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Comparison of anemophilous (wind-pollinated) and entomorphilous (insect-pollinated) flowers

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Comparison of anemophilous (wind-pollinated) and entomorphilous (insect-pollinated) flowers

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12.4.1 Fertilization & Development in Flowering Plants pollen grains send out pollen tubes which grows down the style & ovary, towards the micropyle pollen grains are attracted by sugars in stigma and secrete enzymes to digest a pathway through the style

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Double Fertilization: male gamete goes into the ovule and fertilizes with the egg cell the other male gamete fuses with the polar nuclei to form the triploid endosperm

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12.4.2 Methods of Preventing Inbreeding

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Self-pollination: the transfer of pollen from the anther to the stigma of the same flower, or of another flower of the same plant Cross-pollination: the transfer of pollen to a flower on a different plant of the same species *If pollen lands on the stigma of a plant of a different species, it usually dies. Since cross-pollination results in a great variability of more adaptable offspring, many plants prefer cross-pollination 12.4.2 Methods of Preventing Inbreeding

Methods to prevent self-pollination: 

Methods to prevent self-pollination

Methods to prevent self-pollination: 

Methods to prevent self-pollination

Methods to prevent self-pollination: 

Methods to prevent self-pollination

Methods to prevent self-pollination: 

Methods to prevent self-pollination

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12.4.3 Development of Fruits and Seeds Fate of floral parts after fertilization

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12.4.3 Development of Fruits and Seeds Fate of floral parts after fertilization

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12.4.3 Development of Fruits and Seeds Fate of floral parts after fertilization

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12.4.3 Development of Fruits and Seeds Fate of floral parts after fertilization

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12.4.3 Development of Fruits and Seeds Fate of floral parts after fertilization

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12.4.3 Development of Fruits and Seeds Fate of floral parts after fertilization

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12.4.3 Development of Fruits and Seeds Fate of floral parts after fertilization

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12.4.3 Development of Fruits and Seeds Fate of floral parts after fertilization

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12.4.3 Development of Fruits and Seeds Fate of floral parts after fertilization

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12.4.3 Development of Fruits and Seeds Fate of floral parts after fertilization

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12.4.3 Development of Fruits and Seeds Fate of floral parts after fertilization

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12.4.3 Development of Fruits and Seeds Fate of floral parts after fertilization

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The most common food stores in seeds is carbohydrates (in the form of starch). Many young seeds store sugar but this changes to starch as they mature. Lipid are often stored in the cotyledons and may form a high percentage of the dry weight, e.g. peanuts. Proteins are found to a lesser extent in seeds but wheat has an aleurone layer and protein is stored in the cotyledons of legumes and nuts.

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A fruit is an ovary after fertilization and it contains seeds Functions: Fruits: protect & disperse the embryo Seeds: protect the embryo Provide food for the embryo Help in dispersal of the plant

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Fruits and Seeds Dispersal Dispersal of seeds by fruits – (1) by wind, (2) by animals, (3) by water, (4) by mechanical force, (5) Censor mechanism, and (6) Casual mechanism. Significance of fruits & seeds dispersal It prevents overcrowding & competition for limited resources. It allows the plants to have their seeds to land on suitable places for germination, thus increases the chance for the plants to colonize new areas.

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Acer劍葉槭 Clematis甘草藤

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By animal: juice fruits With hooks: Burdock牛旁 With hooks: Bur-marigold鬼針草 With hooks stick onto our clothes

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1. Wind Dispersal - fruits are light and have a large surface area to catch wind examples: sycamore (pericarp extended to form a wing), dandelion (hairy parachute) 2. Animal Dispersal fruits possess hooks or spines to cling to animals bodies fruits as food for animals: 'stoned seeds' remain intact & pass out with the animals' faeces, e.g. apple, cherry

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3. Water Dispersal - fruits like coconut contain air space which makes the fruit buoyant - seed is covered in a spongy or fibrous layer 4. Mechanical Dispersal - pods with a leathery skin which splits open to throw seeds away over a fairly wide area, e.g. pea

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5. Censer Mechanisms fruits are borne at the ends of long stalks with holes through which seeds are shaken as they blow in the wind; pores are closed in wet conditions, e.g. poppy 6. Casual Mechanisms taking opportunities & using any available means of dispersal, e.g. acorns (from oak tree) may be rolled along the ground by wind or carried about by squirrels or float downstream in rivers

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The Structure of Seeds

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12.4.4 Dormancy The water content of seeds is very low (5-10% weight) and is the major factor in preventing germination. Periods of dormancy may last for a number of years. Some seeds fail to germinate for one reason or another: 1. Light is necessary for germination of certain seeds 2. A sustained period of cold is needed to make some seeds of temperate climates germinate to ensure their seeds do not germinate in dry autumn & cold winter 3. Some seeds need the heat of a flash-fire for germination

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4. Time is needed for maturity / internal chemical changes to complete before germination 5. Seed coat impermeable to water &/or gases; time for decay of seed coat; some seeds need physical abrasion or partial digestion in intestine to break dormancy 6. Presence of natural chemical inhibitors

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12.4.5 Germination & Early Growth in Flowering Plants Conditions necessary for seed germination 1. Water - taken up through the micropyle; softens seed coat; cotyledons swell up & burst through seed coat; enzymes are activated to break down starch to simple sugars and proteins to amino acids; fats are converted into fatty acids & glycerol; these dissolve in water & are transported to the growing points of the embryo

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Glucose, fatty acids & glycerol provide respiratory substrates from energy for growth is released; Glucose is also used in the formation of cellulose cell walls; Amino acids are used to form new enzymes and structural proteins within new cells.

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2. Temperature – warmer temperatures enable enzymes to work 3. Oxygen - enables seeds to respire aerobically to supply energy for growth

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Epigeal: growth of the cotyledon above soil Hypogeal: growth of the cotyledon below soil