Abstract
1. The mode of arrangement of chromosomes in the prophase of heterotypic division in the pollen mother-cells of Rumex acetosa, L. is telosynaptic.
2. After the stage which is likely to be regarded as the second contraction, six small loops and a large loop make their appearance. The small loops are bivalent chromosomes, while the larger one which corresponds to the tripartite chromosome described by KIHARA and ONO is trivalent; that is, it consists of three parts attached end to end, the middle part being the largest. The middle and two side parts of a tripartite chromosome were called M, m1 and m2 respectively by the authors just above named. There is some evidence for the view that the fifteen univalent chromosomes making up the seven loops may arrange themselves attached end to end in the early univalent spireme.
3. Each of the six small bivalent chromosomes in the metaphase, when they are condensed into short rods indicating in general no double nature, separates usually into two univalent chromosomes, while the tripartite chromosome takes various forms in later stages of prophase, and enter the nuclear plate with the shape and position of a V. The two arms of the V represent the m1 and m2 and the apex of the V represents the M. In the anaphase the m1 and m2 travel to the one pole and the M to the opposite.
Occasionally the tripartite chromosome seems to take other forms and positions in the metaphase and the early anaphase. The later behavior of the tripartite chromosome in such cases is unknown, though we may expect to find interesting features later on.
4. In the anaphase the two daughter nuclei have seven and eight chromosomes respectively. They are assumed after homoeotypic division to produce two kinds of spores in a tetrad.
5. In consideration of chromosome relations, there will be at least two kinds of pollen grains formed. It was demonstrated in some of the young pollen grains that one grain had seven chromosomes, while others eight.
6. The male individual of Rumex acetosa has fifteen chromosomes, one of which is distinctly larger than the rest. In the female there are fourteen somatic chromosomes. Two of them are considerably larger than the others. The larger chromosomes found in the somatic chromosome garnitures of the male and female plants are to be considered as the M′s derived from the tripartite chromosomes. Owing to lack of material, the maturation divisions of the female could not be sufficiently followed; nevertheless from the facts above mentioned it will be most reasonable to regard the tripartite chromosome as an idiochromosome complex, of which the M represents X chromosome and the m1 and m2 correspond to Y chromosome, as was first interpreted by KIHARA and ONO.
7. The results of my own observations on Rumex acetosa, L. may be said to confirm those of KIHARA and ONO, except that some points on the prophase, the special behavior of the tripartite chromosome, and the formation of different kinds of young pollen grains, were added to the results of these authors.
