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. embryo sac can be "easily", if at all possible, be concluded from Polygonum-type 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. 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). Strasburger was the first to study such embryo sac. A large vacuole is formed between the nuclei. 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. Variations found in the remaining 30 percent represent derivations from the Polygonum type of seed development. In the monosporic, or Polygonum-type embryo sac, meiosis of the diploid megaspore mother cell in the nucellus produces four haploid megaspores. Initially, the megaspore cytoplasm is non-vacuolated but later small vacuoles appear which fuse to form a large vacuole. It develops from chalazal functional megaspore (4th from micropyle). 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. The embryo sac is formed from the chalazal megaspore in the tetrad and is eight-nucleate. Since this embryo sac develops from one megaspore, that is why this is monosporic embryo sac. One of which gives rise to the embryo sac. The functional megaspore enlarges. Based on the number of megaspores, embryo sacs can be divided into three types: monosporic, bisporic, and tetrasporic (Web Figure 21.3.A). Nucleus of functional megaspore divides by three mitotic divisions to form 8 nuclei, out of which Polygonum Type - Monosporic Embryo Sacs. These two nuclei again divide so that the number of nuclei increases to four. The development of the embryo sac begins with elongation of the functional megaspore. pblygonum- [2] [3] The bisporic and tetrasporic patterns undergo varying processes and result in varying embryo sacs as well. 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. Therefore, it is also called as Polygonum type. The normal type of embryo sac development has been studied in Polygonum by Strasburger. This embryo sac has four well-defined megaspores. Thus the daughter nuclei move to the micropylar and chalazal poles of the embryo sac. 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