Abstract/Summary

In ultrathin sections of 7- and 8-d-old anther cultures of Nicotiana tabacum examined in the electron microscope, embryogenic pollen was distinguished by the presence of zones in the cytoplasm of the vegetative cell consisting of multivesiculate bodies resembling lysosomes. These zones increased with time, and in 12-d-old samples (in which divisions of the vegetative cell were about to commence), were almost devoid of contents. Ribosomes appeared to be virtually eliminated from the cell and many other organelles degraded; the few that remained, mainly plastids, were clustered around and in close contact with the vegetative nucleus. Plastid contents also declined with time, but mitochondrial structure changed little. It is concluded that the gametophytic cytoplasm is destroyed before the first sporophytic divisions of the vegetative cell. In the same sections, other structurally viable pollen which lacked lysigenous zones was characterized by dense vegetative cytoplasm containing numerous small vacuoles. Mitochondrial and plastid structure differed from that of the embryogenic grains, and in some instances lacked clear structural definition. Extensive stacks of rough e.r., which traversed the cytoplasm of the vegetative cell, were interpreted as assembly sites for excess gametophytic protein and RNA synthesised during the culture period.