Electron-microscope and histochemical procedures were used to study the development and breakdown of the chalazal proliferating tissue in Capsella. This tissue is formed by the enlargement of several layers of nucellar cells at the chalazal end of the embryo sac. When the embryo reaches the early globular stage these enlarged cells start to disintegrate, beginning with those immediately bordering the embryo sac and continuing until all have broken down. Characteristic ultrastructural changes accompany the development and breakdown of the chalazal proliferating cells. The mature cells form increased numbers of dictyosomes and large amounts of endoplasmic reticulum (ER). This is accompanied by a thickening of the cell wall. As the cells begin to break down, electron density increases, plastids become polarized in the cells, dictyosomes disappear and the ER is dispersed and fragmented. Plastids, some mitochondria, and pieces of ER appear to be digested in autophagic vacuoles. Cell disorganization is accompanied by an increased number of microbodies and multivesicular bodies per cell. Finally, the nucleus breaks down and the plasmalemma disappears. The end wall ruptures and releases intact mitochondria, ribosomes, and portions of degenerated cytoplasm into the endosperm. Histochemical changes accompany these events. Also discussed are the antipodals and the destruction of the proximal part of the chalazal nucellus by the expanding megagametophyte prior to the development of the chalazal proliferating tissue.

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