1. Seven varieties of rice plants have been cytologically examined. The chromosome number in each is determined as 12 haploid, 24 diploid. Satellites and secondary constrictions are not found in the chromosomes.
2. In the somatic prophase a pair of chromosomes with terminal knob-like bodies remain attached by the knob to two Small nucleoli or one large nucleolus which results by the fusion of the two. These knobs must be concerned in the growth of the telophase nucleoli.
3. The resting nucleus of the pollen mother-cells generally has a single nucleolus with no crystalline bodies.
4. The leptotene thread is split like the somatic early prophase chromosomes, the two chromonemata being closely twisted about each other.
5. During the heterotypic prophases the nucleoli, one or two in number, remain closely connected with a particular pair of chromosomes at one end (referred to as the nucleolar body) from the leptotene stage until the nucleolus disappears completely at metaphase.
6. During synizesis the nucleolus in some of the nuclei of the varieties studied produces a bud, which grows until the two nucleoli attain the Same size at early diakinesis by transfer of material from the larger to the smaller.
7. Comparison of six varieties of rice shows no difference in the size of the nuclei and nucleolar contents. But it shows a significant difference in the size of the nuclear diameter in those cells with one nucleolus at diakinesis and with two (Table 1). This is correlated with the differences in the size of the nucleolar diameter when one nucleolus or two are present at diakinesis. Only the larger nucleoli (which are present in the larger nuclei) bud, and the “budding” takes place after the maximum size of the nucleolus is reached.
8. Budding of the nucleolus may be due to the relative increase of the nucleolar material within the mother nucleolus or it may be an artefact produced by the treatment. When budding takes place it is always at the point of attachment of the terminal chromosome knob.
9. The method of chromosome pairing is parasynaptic, pairing commencing at the ends of the chromosomes. Variation in size and shape of the 12 bivalent chromosomes is found in early diakinesis, one larger twisted pair being particularly noticeable.
10. From resting stage until early diakinesis there is an increase in the size of the nucleolus, followed by a gradual decrease from late diakinesis with the gradual condensation of the chromosomes until the time of its complete dissolution at metaphase.
11. It is concluded that the nucleolus contributes matrix substances (not chromatin) indirectly to all the chromosomes, which is carried over by metaphase and anaphase chromosomes, and a definite nucleolus is organized at telophase from this substance under the influence of the nucleolar body present an the end of one chromosome of the haploid complement. Trivalent and univalent chromosomes are sometimes found at diakinesis in certain Japanese varieties.
12. The phenomena of cytomyxis, pollen mother-cell fusion, binucleate and tetraploid pullen mother-cells and failure of cytokinesis during meiotic divisions of some of the microsporocytes, are described. As the result of these aberrations, diploid gametes are formed, which will account for the origin of triploid and tetraploid plants of rice.
13. The origin of the nucleolus is discussed and its relation to the nucleolar body; also the possible rôle of the nucleolus in inheritance.
14. It is emphasized that further studies of a biochemical nature are required before definite conclusions may be drawn as to the relationship between the matrix substance of the ehromosomes and the nucleolus.
This work was carried out in the Botanical Department of King's College, University of London. The author wishes to express his indebtedness and sincerest gratitude to Professor R. Ruggles Gates, under whose supervision this work was done
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