A Cytological Study of <i>Oryza sativa</i> L.

Abstract

1. Five races of rice plant have been cytologically examined, and the chromosome number is found to be the same in all of them, the haploid number being 12.2. The resting nucleus of the pellen mother-cells generally has a single nucleolus with one or more vacuoles and crystalline bodies.3. Two firmly and permanently attached nucleoli are seen during the early prophase stage in three of the races. The other two have a single large nucleolus each.4. The synizetic knot in most Gases is very compact and the threads are extremely delicate and not distinct.5. No continuous spireme is formed at the loosening of the synizetic knot, but the reticulate character of the threads is retained until later.6. The single or double nucleolus is attached to the thread at more than one point.7. Dark stained bodies are seen scattered all over the reticulate thread. Chromatin is aggregated around these bodies, the thread is constricted and the chromosomes are formed.8. The secondary nucleolus disappears during the development of the chromosomes, and the primary one remains spherical all through diakinesis.9. At late diakinesis the primary nucleolus fades away gradually until it disappears; at the same time fibrilla-like structures appear between the chromosomes.10. Comparison of the five varieties of rice studied, shows that although they all have the same size of nuclei and the same number of chromosomes, yet they differ characteristically in the nucleolar content of their pollen mother-cells. These differences are correlated with the absolute and relative size of the nucleoli, and also with the time when the secondary nucleolus is budded off from the primary (see Table 2).11. The above facts suggest that when the nucleolus divides it separates two different materials; and Supports the view that the secondary nucleolus contributes material to the chromosomes, while the primary may contribute to the spindle.12. No resting nuclei are formed during interkinesis; and at homotypic telophase twelve chromosomes could be distinguished.The writer wishes to express his sincerest gratitude to Professor R. RUGGLES GATES of King's College, University of London, under whose supervision this work was carried out, for his kindly advice and valuable criticism.