Notes And Study Material for Exam Papers

Difference Between Polyembryony and Parthenocarpy Polyembryony: In several genera of angiosperms, more than one viable embryo in the seed has been observed. It is more frequent in Gymnosperms than in angiosperms. This condition arises in various ways: The oospore produces more than one embryo during development as a result of splitting. This is known as cleavage polyembryony and is very common in gymnosperms (e.g. Pinus) and angiosperms e.g. Allium, citrus, and some orchids. An ovule may contain more than one functional megaspore producing as many embryo sacs and oospheres which on fertilization give rise to more than one embryo in the seed. In addition to the normal embryo produced from a fertilized oosphere, the embryo may also be formed from the synergids (e.g. Dandelion) or rarely the antipodals with or without fertilization. (iv) Additional embryos may be produced as a result of saprophytic budding by the division of the cells of the nucellus (e.g. lemon) or in

Development of Endosperm It begins before that of the embryo, arises from the primary endosperm nucleus (also known as triple fusion nucleus) formed as a result of the triple fusion of three nuclei i.e. two polar nuclei and a male nucleus. The primary endosperm nucleus is triploid (3N). It undergoes a series of divisions resulting in the formation of many free nuclei lying within the embryo sac. A vacuole appears in the center so that the nuclei are pushed to the periphery. Later on, unless the endosperm is absorbed by the embryo in the free nuclear stage, wall formation takes place starting from the periphery towards the center of the embryo sac, and by further centripetal growth, the embryo sac is filled with a compact cellular tissue around the embryo without any intercellular spaces. This tissue is endosperm occasionally as in coconut a vacuole persists in the center of the embryo sac as it is not completely filled by the endospermic tissue. This type of endosperm development s

An ovule (megasporangium) may arise from the placenta at the base of the ovary or on the inner surface of the ovary. It is borne on a distinct stalk, the funicle which is attached to the body of the ovule at a point called the hilum. The central and somewhat conical part of the ovule is the nucellus. It consists of a mass of parenchymatous cells. It represents the metasporangium proper and is invested by two-layered integuments in monocotyledons and primitive dicotyledons (Archicliamydeae) but by single-layered integument in the higher dicotyledonous families (metachlamydeae); very rarely unitegmic and bitegmic ovules occur in the same family. In many cases, single integument has resulted due to the fusion of two separate primordials or by elimination of one of the two integuments. The third integument in the form of aril is found in plants like Asphodehus and Trianthema, while Ricinus and several other plants of the family, Euphorbiaceae, show an integumentary proliferation called

A cross section of young anther shows that each anther lobe contains two pollen sacs or microsporangia but the number may vary among different angiosperms from one to many. As a rule microsporangia extend as longitudinally running sacs through the entire length of the anther. Later on as the anther matures, the intervening tissue breaks down between each pair of pollen sacs and the two pollen sacs coalense into one, thus forming two large cavities instead of four in each anther. In Malvaceae anthers are one called (monotherous) as there are two instead of four pollen sacs per anther and these also coalesce at maturity to form single loculus. Very rarely four celled sacs may remain separate in a mature anther. Within each pollen sac of an angiosperm as within a Microsporangium of Salaginella or Pinus are produced small numbers of micropyle mother cells surrounded by a conspicuous layer of nutritive cells, the tapetum outside the tapetum is the wall of the sporangium consistin

An ovule (megasporangium) may arise from the placenta at the base of the ovary or on the inner surface of the ovary. It is borne on a distinct stalk, the funicle which is attached to the body of the ovule at a point called the hilum. The central and somewhat conical part of the ovule is the nucellus. It consists of a mass of parenchymatous cells. It represents the metasporangium proper and is invested by two-layered integuments in monocotyledons and primitive dicotyledons (Archicliamydeae) but by single-layered integument in the higher dicotyledonous families (metachlamydeae); very rarely unitegmic and bitegmic ovules occur in the same family. In many cases, single integument has resulted due to the fusion of two separate primordials or by elimination of one of the two integuments. The third integument in the form or aril is found in plants like Asphodehus and Trianthema, while Ricinus and several other plants of the family, Euphorbiaceae, show an integumentary proliferation calle

Development of endosperm begins before that of embryo. It arises from the primary endosperm nucleus (also known as triple fusion nucleus) formed as a result of triple fusion of three nuclei i.e. two polar nuclei and a male nucleus. Primary endosperm nucleus is triploid (3N). It undergoes a series of divisions resulting in the formation of many free nuclei lying within the embryo sac. A vacuole appears in the centre so that the nuclei are pushed to the periphery. Later on unless the endosperm is absorbed by the embryo in free nuclear stage, wall formation takes place starting from the periphery towards the centre of embryo sac and by further centripetal growth, the embryo sac is filled with a compact cellular tissue around the embryo without any intercellular spaces. This tissue is endosperm occasionally as in coconut a vacuole persists in the centre of the embryo sac as it is not completely filled by the endospermic tissue. This type of endosperm development since it involves the

In several genera of angiosperms more than one viable embryo in the seed has been observed. It is more frequent in Gymnosperms than in angiosperms. This condition arises in various ways: (i) The oospore produces more than one embryo during development as a result of splitting. This is known as cleavage polyembryony and is very common in gymnosperms (e.g. Pinus) and angiosperms e.g. Allium, citrus and some orchids. (ii) An ovule may contain more than one functional megaspore producing as many embryo sacs and oospheres which on fertilization give rise to more than one embryo in the seed. (ii) In addition to the normal embryo produced from a fertilized oosphere, embryo may also be formed from the synergids (e.g. Dandelion) or rarely the antipodals with or without fertilization. (iv) Additional embryos may be produced as a result of sporophytic budding by the division of the cells of the nucellus (e.g. lemon) or integument (e.g. onion). Parthenocarpy: As a rule