A Cytological Study of Oryza sativa L

With Plate VIII

Abstract

Maiosis in the development of pollen-grains. In the young stage of the pollen-mother-cell, the chromatin-masses, nearly as many as the diploid number of chromosomes, are scattered in the nuclear cavity, each two forming a pair.
The chromatin-masses stretch out into the double threads somewhat with an appearance of pearl-strings.
They enter the chief contraction stage of synapsis. But the double nature of the threads can be still clearly made out in thin sections cut 2μ thick.
The conglobed threads begin to become loose and at last stretch themselves over the nuclear cavity, where the double threads unite together to form simple threads. Thus the conjugation of chromatin-threads does not take place in the contraction stage itself, but after this stage has passed over.
After a while the separation of the united threads takes place. Then the segmentations occur, and 12 double segments or gemini make their appearance.
Thus through all stages of prophase, the paired arrangement or union of chromatin-threads or chromosomes is formed by parallel association, not by an end-to-end association, viz. the process is a parasyndesis, not a metasyndesis.
They become shorter and thicker, and assume a dumb-bell shape or a somewhat square shape.
In the prophase of the division the nucleus is usually provided with several dwarf-nucleoli beside the ordinary large one.
The behavior of chromosomes in the homotype division is very remarkable. Some of them present paired arrangements and form pseudo-gemini and they show even a tendency to form a group of more than two.
An abnormal case in the heterotype division has been observed, the chromosomes are smaller than the ordinary ones and two or three of them, which are especially small, take a rod-shape.
Chromosomes in the somatic cell. The somatic number of chromosomes is 24, but very often we find a larger number. Chromosomes always present paired arrangements. The chromosomes show here the difference of size and shape clearly, while it was not so clear in the maiotic phase.
Embryosac and endosperm-formation. The development of the embryosac is normal. In the antipodals, which consist of 3 cells at first, further cell- or nuclear divisions take place, as is the case with other Gramineae.
The 'double fertilization' takes place in this plant, and the sperm-nucleus reaches the upper polar nucleus.
The regular appearance of the three nucleoli in the young daughter-nuclei of the endosperm-tissue is looked upon as an indication of the autonomy of the three different nuclei, which have given rise to the endosperm-initial, up to a certain stage of endosperm development.
The endosperm-formation proceeds at first as usual with free nuclear divisions, and is followed later by the simultaneous wall-formation. In the further development of the tissue the cell-wall formation is often suppressed, and the fusion of the daughter-nuclei occurs, which results in the formation of syntriploid nuclei.