REPRODUCTION IN PLANTS BIO NOTES Asexual reproduction x Sexual reproduction Sexual Reproduction Asexual reproduction Process involving the fusion of two gametes to form a zygote, producing genetically dissimilar offspring. Advantages: - Offspring may inherit beneficial qualities from both parents - Greater genetic variation in the offspring, leading to species that are better adapted to changes in the environment Disadvantages: - Two parents are required (except plants with bisexual flowers) - Fusion of gametes required - Slower method of producing offspring Process which produces genetically identical offspring from one parent, without the fusion of gametes. Advantages: - Only one parent is required - Fusion of gametes not required - All beneficial alleles passed on - Faster method of producing offspring - Allow organisms to colonise area rapidly Disadvantages - No genetic variation in the offspring - Species not well adapted in the environment Parts of a flower Petal Modified leaves Insect pollinated flowers are bright and large enough for insects to stand on All the petals together make up the corolla Sepal Modified leaves Enclose and protect other parts of flower in bud stage All the sepals together make up the calyx Stamen and carpel Stamen is the male parts of flower (filament and anther) Carpel or pistil is the female parts of flower (stigma, style and ovary) Receptacle Enlarged end of flower stalk (or pedicel) which bears the other parts of flower Anther Each anther contains two lobes and each lobe consist of pollen grains Contains vascular bundle Filament Stalk that holds the anther in a suitable position to disperse the pollen grains. Ovary Structure that develops into a fruit after fertilisation Produces and protects ovules Ovule Structure that develop into a seed after fertilisation Produces ovum by meiosis Ovum is hence haploid Style Stalk that connects stigma to the ovary Holds the stigma in a suitable position to trap pollen grains Pollen grains Produced by meiosis, hence are haploid Each contains a male gamete Inflorescences A cluster of flowers borne on the same stalk Pollination - Transfer of pollen grains from the anther to the stigma from female flower to male flower. - Self-pollination; Transfer of pollen grains from the anther to the stigma of the same flower or of a different flower on the same plant. - Cross-pollination: Transfer of pollen grains from one plant to the stigma of a flower in another plant of the same species. Self-pollination Cross-pollination Features Flowers are bisexual with anthers and stigmas maturing at the same time Stigma is situated directly below the anthers In some plants, some flowers never open, so that only self-pollination occurs Dioecious flowers (male and female flower on different plants) so that they cannot undergo self-pollination Anthers and stigmas may mature at different times Stigmas and anthers of a bisexual flower are separated some distance away Advantages Only one parent is required Beneficial qualities are more likely to be passed down to offspring Not dependent on external factors such as insect/wind pollination Higher probability of pollination Offspring may receive beneficial qualities from both parents Greater genetic variation, increases the chance of species surviving changes in environment Higher chance of heterozygous plants Disadvantages Less genetic variation in offspring, Two parent plants are required Depends on external factors for pollination (insect or wind pollination) Lower probability of pollination than self-pollination More energy and pollen wasted Less adaptive to changes in the environment Insect-pollinated flower — Clitoria Structure - Calyx: Consists of five green sepals - Epicalyx: Base of calyx enclosed by two green leaves (epicalyx) - Corolla: Consists five brightly-coloured petals of different shape and sizes - Standard petal: Large dark blue petal that has nectar guides - Wing petals: Two smaller dark blue petal enclosed by standard petal - Keel petals: Two small yellow green petal enclosed by wing petals It encloses the reproductive organs - Stamens: Nine stamens fused along their length, the tenth stamen is free - Carpel: Stigma is small and compact, located at end of style The style is a long, curved, hairy structure Ovary is long and narrow, with single row of ovules Pollination of Clitoria - 1. Insect lands on standard petal and follows the nectar guides into the flower. - 2. Insect forces its way in between the two wing petals and moves in to collect nectar. - 3. Insect’s back forces keel petals upwards, exposing stigma and anthers. - 4. Stigma and anthers brush against the hairy back of insect, causing pollen grains to stick on the hairy back of insect. - 5. At the same time, pollen grains from another flower on back of insect are transferred to sticky stigma. - 6. Insect leaves the flower after some time, then the keel petals spring back to its original position, enclosing anthers and stigma again. Wind-pollinated flower — Ischaemum muticum Structure - Flowers: Flowers occur in pairs - Upper flower: Bisexual, consists of 1 ovary, 2 feathery stigmas and lodicules, 3 stamens Both lodicules swell to force flowering bracts open so that anther and stigma can emerge - Lower flower: Male, consists of 3 stamens and 2 lodicules - Spikelet: Each pair of flowers with a short stalk forms a spikelet Each spikelet is protected by non-flowering bracts Pollination of Ischaemum muticum - 1. Mature stamens are long, pendulous, and hang out of the bracts, exposed to wind. - 2. When blown by wind, the dust-like pollen is shaken free and dispersed by wind. - 3. Mature stigmas project out of bracts and are large, extended and feathery, in order to provide large surface area to receive any pollen floating in the wind. Differences between insect-pollinated and wind-pollinated flowers Features Insect-pollinated Wind-pollinated Petals Flowers are large and brightlycoloured petals Small and dull-coloured, no petals Nectar Present Absent Scent Fragrant and sweet-smelling Absent Stigmas Small, compact and do not protrude out of flower Large, feathery and protrude out of flower (to trap pollen grains) Stamens Not pendulous and do not protrude out of flower Long, pendulous and protruding anthers Pollen Fairly abundant Larger with rough surfaces so that they can readily cling on insects More abundant Tinier with smooth surfaces so that they are easily blown in the wind Nectar guide Nectar guides may be present Nectar guides absent Fertilisation - 1. After pollination, the pollen grain germinates after it comes into contact with stigma, in response to the sugary fluid secreted by the mature stigma. - 2. A pollen tube grows out from each pollen grain, down the style and eventually into the ovary. The tip of the pollen tube contains two male gametes. - 3. As pollen tube grows, it secretes enzymes to digest the surrounding tissue of the stigma and style. - 4. The pollen tube enters the ovule through the micropyle. - 5. In the ovule, the tip of pollen tube absorbs sap and bursts, releasing the two male gametes. - 6. The nucleus of one male gamete fuses with the nucleus of the ovum to form the zygote.
