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J Integr Plant Biol, 1999, 41 (12): -, Research Article
Ultrastructural Studies on Megaspore Formation in Ginkgo biloba
JI Cheng-Jun, YANG Xiong and LI Zheng-Li (Lee C L)
doi:
Abstract
The megaspore mother cell of G/nkgo biloba L. became elliptical before the initiation of meiosis with its nucleus migrating towards the micropylar end and surrounded with starch-bearing plastids. Mitochondria were distributed at the chalazal end. Large amount of the rough endoplasmic reticulum (RER) accumulated at the micropylar end but mitochondria and plastids were hardly visualized there. At the prophase I of meiosis, vacuole enlarged, endoplasmic reticulum (ER) at the micropylar end decreased and mitochondria and plastids became indistinctive. The dyad, formed after the first division of meiosis, demonstrated more substantial polar differentiation, in which the cell towards the micropylar end was relatively smaller and contained looped ER and sparsely arranged mitochondria but almost no plastids. In contrast, the cell towards the chalazal end became enlarged with enriched organelles. During the second division of meiosis, the cell towards the chalazal end (B) divided ahead of the cell towards the micropylar end (A). After the formation of the linear tetrad, the cell towards the cbalazal end (B2) which was the largest of all, became the functional megaspore. The other three cells (Al, A2 and B1 ) began to degenerate at quite different time. A1 and Aa cells degenerated first. Later, a thick wall formed between B1 and B2. Owing to the change of polarity during meiosis, aside from the conventional formation of linear tetrads, T-shaped tetrads (Al ,A2 horizontally lain and B1, B2 cells vertically lain) or linear triads (Al, A2 and an undivided B cell which later became the functional megaspore) were infrequently observed.
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