class-xii subject: biology chap 2: sexual reproduction in … · 2020. 5. 1. · microsporogenesis...
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Class-XII Subject: Biology
Chap 2: Sexual Reproduction in Flowering
Plants
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Microsporogenesis
Pollen grain
Pistil, Megasporangium & Embryo Sac
Megasporogenesis
Female gametophyte
Pollination- Kinds and Agents
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ovule Typical anatropous ovule 19-Apr-20 10
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By Wind (Anemophily) By Water (Hydrophily)
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By insects (entomophily) Differentiate
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Either pollen is released before the stigma becomes receptive or vise versa.
Anther, stigma placed at different position
Self-incompatibility-to prevent inbreeding
Production of unisexual flowers- castor & maize prevents autogamy but not geitonogamy. In papaya male and female flowers are present on different plants; this condition prevents both autogamy and geitonogamy.
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Process of removing anthers/stamens in a bisexual flower of a plant to be used as the female parent in a cross is called Emasculation. After this ,the flower contain only female reproductive organ so that the desired pollen from male plant will be provider by breeder. By: 1. Hand Emasculation 2. Emasculation by hot water,
cold water ,alcohol treatment
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Polythene bag is put up to prevent contamination of its stigma with unwanted pollen
When the stigma of bagged flower attains receptivity, mature pollen grains collected from anthers of the male parents are dusted on the stigma and the flowers are rebagged.
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Pollen tube releases 2 male gametes into the cytoplasm of the synergid.1male gamete+egg cell= snygamy.Other male gamete+ 2 polar nuclei=PEN(triple fusion).
The central cell after triple fusion becomes the primary endosperm cell(PEC) and develops into the endosperm while the zygote develops into an embryo.
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Development of endosperm from primary endosperm nucleus in the central sac of embryosac.
Development of embryo from diploid zygote
Development of seed from ovule
Development of fruit from ovary.
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Apomixes Polyembryony
(apo= without;mixis=mixing).Type of asexual reproduction that resembles sexual reproduction. Seeds are produced by without fertilisation. Hence, all the individuals produced apomictically are genetically similar to the parent producing them.
1.Non-recurrent apomixis (haploid parthenogenesis)
2.Recurrent apomixis (diploid parthenogenesis)
Occurrence of more than one embryo in a seed(discovered in orange by Leeuwenhoek in 1719)
1.Simple polyembryo-more than one embryo sac, so more than one egg are present.eg.Brassica
2.Mixed polyembryo If the same embryo sac, give
rise to many embryos, it is known as true polyembryony. If several embryos are formed from cells other than those of the embryo sac, it is known as false polyembryony.
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