The normal type of embryo sac development has been studied in Polygonum by Strasburger. Other articles where Polygonum type is discussed: angiosperm: Ovules: …megasporogenesis and megagametogenesis, called the Polygonum type, occurs in 70 percent of the angiosperms in which the life cycle has been charted. Therefore, the resulting embryo sac is a seven-celled structure consisting of one central cell, one egg cell, two synergid cells, and three antipodal cells. The functional megaspore enlarges. Its nucleus divides. pblygonum- It develops from chalazal functional megaspore (4th from micropyle). Based on the number of megaspores, embryo sacs can be divided into three types: monosporic, bisporic, and tetrasporic (Web Figure 21.3.A). Thus the daughter nuclei move to the micropylar and chalazal poles of the embryo sac. (a) Polygonum type – Polygonum type is the normal type of development of embryo sac. One of which gives rise to the embryo sac. Polygonum Type - Monosporic Embryo Sacs. This embryo sac has four well-defined megaspores. An eight nucleated and seven celled embryo sac formed from a single megaspore is called polygonum type. embryo sac can be "easily", if at all possible, be concluded from Polygonum-type embryo sac. Variations found in the remaining 30 percent represent derivations from the Polygonum type of seed development. Its cells are polyploid (triploid or pentaploid, depending on the embryo sac type). Therefore, it is also called as Polygonum type. Nucleus of functional megaspore divides by three mitotic divisions to form 8 nuclei, out of which A large vacuole is formed between the nuclei. Approx- imately 70% of the species examined, including Arabidopsis and maize, have this form of embryo sac (Russell, 1978; Mansfield et ai., 1990; Webb and Gunning, 1990). [2] [3] The bisporic and tetrasporic patterns undergo varying processes and result in varying embryo sacs as well. Since this embryo sac develops from one megaspore, that is why this is monosporic embryo sac. The embryo sac is formed from the chalazal megaspore in the tetrad and is eight-nucleate. Assertion : 7-celled, 8 nucleate and monosporic embryosac is called polygonum type of embryo sac
Reason : It was discovered by Hofmesister for the first time in polygonum. In this type the nucleus of functional megaspore divides to form two nuclei and moves apart: one towards the micropylar end and the other towards the chalazal end. Development of the Polygonum-Type Embryo Sac The Polygonum-type pattern illustrated in Figure 2 is the most commonly observed form of embryo sac development. Initially, the megaspore cytoplasm is non-vacuolated but later small vacuoles appear which fuse to form a large vacuole. In the monosporic, or Polygonum-type embryo sac, meiosis of the diploid megaspore mother cell in the nucellus produces four haploid megaspores. Strasburger was the first to study such embryo sac. The embryo is small (usually less than one quarter of seed volume), axile (radially sectioned), linear (longer than broad) or rarely rudimentary (tiny relative to endosperm) depending on placentation type, and straight, bent, curved or curled at the upper end. 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