NUCLEAR DIVISIONS IN THE TAPETAL CELLS OF GALTONIA CANDICANS

Abstract

The increasing number of descriptions of nuclear divisions in tapetal cells has established the fact that these divisions are mitotic rather than amitotic, as was previously considered by many to be the case. Strasburger (I882) described typical mitoses of tapetal nuclei. Duggar (I899) also early described mitoses in the tapetal cells of Bignonia venusta, which are normally binucleate during the stages of microsporogenesis following synizesis. Tischler (I906) described mitotic division of the nuclei in tapetal cells of Ribes. The second nuclear division, however, he found to be amitotic. He also found nuclei with the tetraploid chromosome number and concluded that such nuclei resulted from a fusion of the two diploid nuclei formed after the first division. Winkler (i906) described a fusion of nuclei in the tapetal cells of Wikstroemia indica. Mitotic divisions occur in the tapetal cells, resulting in a 2-6 nucleate condition. These nuclei usually fuse to form giant nuclei, but in some cases they remain separate. Tahara (I9IO) found in Morus indica that during the early stages of the heterotypic division the tapetal cells are binucleate. Their nuclei may fuse to form a single tetraploid nucleus which in turn may divide mitotically, or they may divide separately to form four nuclei. The four nuclei then fuse in pairs, so that each cell contains two tetraploid nuclei which may later divide mitotically. Bonnet (I912) made an extensive study of the tapetal cells in several genera of angiosperms and found mitosis to be the only method of nuclear division in the material examined. In Yu,cca the first tapetal nuclear division occurs before synizesis; in Fuchsia it occurs after synizesis. The second divisions are simultaneous in each cell. In some cases two of the spindle poles converge, so that during the telophases two of the four daughter nuclei fuse to form a single tetraploid nucleus. Thus the cell contains one tetraploid and two diploid nuclei. Bonnet also described a fusion of nuclei in a resting condition which results in the formation of tetraploid and octoploid nuclei. Gates and Rees (192I) found only mitotic nuclear divisions in the tapetal cells of Lactuca. The first division occurs at the start of synizetic contraction, and by the time of maximum synizetic contraction the tapetal cells are all binucleate. Some of them remain binucleate; others undergo a second nuclear division as the pollen mother cells reach the open spireme stage. The