Structure of Ovule
• A typical angiosperm ovule is a small structure attached to the placenta by means of stalk called funicle. The body of the ovule fuses with funicle and the point of attachment is called hilum. • The body of the ovule consists of a mass of parenchymatous cells named nucellus. • Nucellus is surrounded by one (unitegmic ovule, e.g., higher dicots) or two (bitegmic ovule, e.g., monocots and primitive dicots) or multicellular integuments. • The integuments leave a narrow passage known as micropyle at one end of the ovule. • The place of origin of the integuments usually lies at the opposite end of micropyle, termed as chalaza. • Embryo sac or female gametophyte is present in the micropylar half of nucellus. • Depending upon the configuration and orientation of the body of ovule in relation to funiculus there are six types of ovules in angiosperms - orthotropous (erect), anatropous (inverted), hemitropous (half-inverted), campylotropous (body curved), amphiotropous (both body and embryo sac curved), circinotropous (funiculus coiled around the ovule).
Megasporogenesis
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The process of formation of haploid megaspores from the diploid megaspore mother cell is called megasporogenesis. Generally, a single megaspore mother cell (MMC) differentiates in the micropylar region of the nucellus. The megaspore mother cell undergoes meiotic division which results in the production of four haploid megaspores. In majority of angiosperms, only one of the megaspores is functional while the other three degenerate. The functional megaspore develops into the female gametophyte (embryo sac). The formation of embryo sac from a single megaspore is called monosporic development. The female gametophyte or embryo sac contains 8 nuclei but 7 cells-3 Chalaza micropylar, 3 chalazal and one central. Raphe Nucellus The three micropylar cells are known as egg apparatus. The middle Integuments cell of the egg apparatus is called egg which is larger with a central vacuole and a nucleus towards the chalazal end while the remaining Antipodal cells two cells are called synergids. Central cell Two polar Each of the synergids bears a filiform apparatus in the micropylar region Embryo sac nuclei which is a mass of finger like projections of the wall into the cytoplasm. Egg (oosphere) Synergids The three chalazal cells of the embryo sac are called antipodal cells. Vascular strand The central cell is the largest cell of the embryo sac. Hilum The central cell contains two polar nuclei which often fuse to form a Funiculus single diploid secondary or fusion or definitive nuclei. Micropyle
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Fig.: Structure of a typical ovule (anatropous ovule) prior to fertilisation.
1. 2.
What are the functions of tapetum? In which type of ovule funiculus is coiled around the ovule? Give an example.
POLLINATION
• The transfer of pollen grains from the anther to the stigma is called pollination.
Types of Pollination Self pollination Autogamy • Transfer of the pollen grains from the anther to the stigma of the same flower.
Geitonogamy • Transfer of pollen grains from the anther to the stigma of another flower of the same plant.
Cross pollination or Xenogamy
• Transfer of pollen grains from anther to the stigma of a genetically different plant.
• Some plants such as Oxalis, Viola and Commelina produce two types of flower - chasmogamous flowers with exposed anthers and stigma and cleistogamous flowers which do not open of all. • Cleistogamous flowers are invariably autogamous as there is no chance of cross-pollen landing on the stigma. 60
MT BIOLOGY
TODAY | MAY ‘18