sexual Reproduction

• Self-incompatibility prevents self-pollination in self-incompatible species.
• Embryos of monocotyledons have a single cotyledon.
• Cell division during pollen grain formation and pollen mother cell formation occur during these events.
• Mature dicotyledonous embryo has two cotyledons and an embryonal axis with epicotyl and hypocotyl.
• The zygote is dormant for sometime in a fertilised ovule due to the presence of two polar nuclei.
• Pollination is the mechanism to transfer pollen grains from the anther to the stigma.
• The developing embryo passes through different stages such as the proembryo, globular and heart-shaped stages before maturation.
• Ovary develops into fruit and ovules develop into seeds.
• Parthenocarpy can be induced through the application of growth substances on certain fruits.
• The products of these fusions are the diploid zygote and the triploid primary endosperm nucleus (in the primary endosperm cell).
• Angiosperms exhibit double fertilisation because two fusion events occur in each embryo sac, namely syngamy and triple fusion.
• Apomicts have several advantages in horticulture and agriculture.
• Some angiosperms produce more than one embryo in their seed, a phenomenon called polyembryony.
• Pollen grains represent the male gametophytes and are found in the opened anthers of Hibiscus or any other flower.
• In over 60 per cent of angiosperms, pollen grains are shed at the 2-celled stage.
• The cytoplasm of pollen grain is surrounded by a plasma membrane.
• The outer three wall layers perform the function of protection and help in dehiscence of anther to release the pollen.
• In the remaining species, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed, this is the 3-celled stage.
• In a transverse section, a typical microsporangium appears near circular in outline and is generally surrounded by four wall layers: the epidermis, endothecium, middle layers and the tapetum.
• The innermost wall layer is the tapetum which nourishes the developing pollen grains.
• Inside each microsporangium several thousands of microspores or pollen grains are formed that are released with the dehiscence of anther.
• Microsporangia develop further and become pollen sacs that extend longitudinally all through the length of an anther and are packed with pollen grains.
• The vegetative cell is bigger, has abundant food reserve and a large irregularly shaped nucleus.
• The generative cell is small and floats in the cytoplasm of the vegetative cell, it is spindle shaped with dense cytoplasm and a nucleus.
• Pollen grains are generally spherical measuring about 25-50 micrometers in diameter and have a prominent two-layered wall: the hard outer layer called the exine and the inner wall called the intine.
• The exine exhibits a fascinating array of patterns and designs.
• The hard outer layer of the pollen grain, the exine, is made up of sporopollenin which is one of the most resistant organic material known.
• As the anther develops, the cells of the sporogenous tissue undergo meiotic divisions to form microspore tetrads.
• Cells of the tapetum possess dense cytoplasm and generally have more than one nucleus.
• The process of formation of microspores from a pollen mother cell (PMC) through meiosis is called microsporogenesis.
• Each cell of the sporogenous tissue is capable of giving rise to a microspore tetrad, making each one a potential pollen or microspore mother cell.
• As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains.
• The microspores, as they are formed, are arranged in a cluster of four cells–the microspore tetrad.
• The inner wall of the pollen grain, the intine, is a thin and continuous layer made up of cellulose and pectin.
• When the pollen grain is mature it contains two cells, the vegetative cell and generative cell.
• Some plants such as Viola (common pansy), Oxalis, and Commelina produce two types of flowers – chasmogamous flowers which are similar to flowers of other species with exposed anthers and stigma, and cleistogamous flowers which do not open at all.
• Agents of Pollination: Plants use two abiotic (wind and water) and one biotic (animals) agents to achieve pollination.
• In Vallisneria, the female flower reach the surface of water by the long stalk and the male flowers or pollen grains are released on to the surface of water.
• Geitonogamy is the transfer of pollen grains from the anther to the stigma of another flower of the same plant.
• Wind-pollination is quite common in grasses.