Evidence for the discontinuity of plastids in the sexual reproduction of a plant

Abstract

Investigation of the fine structure of the egg cell of Pteridium aquilinum , a fern, reveals that the mature egg contains only large amoeboid proplastids, strikingly different from the plastids of the central cell. Early in the life of the egg there is a phase in which numerous vesicular bodies, less than 1 μ in diameter and identified as degenerate plastids, are present in the cytoplasm. As the egg matures these bodies gradually disappear, and simultaneously the nucleus of the egg throws out conspicuous evaginations. Many of these, particularly toward the end of the maturation process, and for about 6 hours after fertilization, are saclike, the contents being in continuity with the nucleoplasm. Arguments are presented for believing that these saclike evaginations become detached, and form plastid initials. Their subsequent development is similar to that of the plastid initials of embryonic cells, and the proplastids so formed replace those eliminated early in the maturation of the egg. This interpretation is held to be compatible with the results of breeding experiments with variegated plants.