Ultrastructural Aspects of the Mature Embryo Sac of Soybean, Glycine max (L.) Merr.

Abstract

Electron-microscopic examination of the embryo sac of soybean, Glycine max (L.) Merr. `Bragg' shows that each cell of the egg apparatus is enclosed by a wall that is thickest at the micropylar end and contains expanded wall areas interspersed between thin areas at the chalazal end. Expanded wall areas are much thicker in the egg cell than in synergids. A filiform apparatus (FA) at the micropylar end of each synergid appears continuous with the embryo sac wall. The surface area of the plasma membrane appressed to the FA is extensive, and abundant endoplasmic reticulum (ER) is associated with the FA. Many plasmodesmata are present in the wall between the egg cell and each synergid, suggesting that the synergids may contribute to egg cell nutrition. Numerous osmiophilic bodies in the egg and synergids indicate that some storage of reserves can occur in these cells. Abundant ER profiles within the egg indicate that it is a relatively active cell. There are few plasmodesmata between the central cell and two synergids, suggesting that the FA contributes less to the nutrition of the central cell than has been proposed for some other species. At the micropylar end of the central cell numerous cytoplasmic organelles are associated with the wall ingrowths During development, the central cell becomes completely filled with packets of starch grains, which mostly disappear before fertilization occurs.