Abstract
1. A natural hybrid Paraixeris denticulate×Crepidiastrum platyphyllum is similar to the artificial hybrid of the same combination.
2. The mode of pairing in these hybrids are characterized by having 5 bivalents regularly, in spite of the morphological dissimilarity of the parental chromosomes. The progeny having aberrant combinations of chromosomes is apt to have 5 bivalents at metaphase, although at earlier stages fewer configurations with multivalents are observed. Separations of chromosome parings in later stages are thought to ocurr at the points of the residual paring.
3. As to the possibility for a natural hybrid to be able to begin a starting line of a new species, the present intergeneric hybrids serve as an example of an occurrance of a new type, since the parental characters are more dissimilar than in ordinary interspecific hybrids.
4. The hybrid Paraixeris denticulata (n=5)×Lactuca squarrosa (n=9) has not 14 chromosomes as expected from the parental chromosomes, but has 10-14. And the majority of the cells have 12 chromosomes. This condition is observed in all the 9 individuals studied.
5. The fragmentation and the elimination are observed in the other hybrids, Paraixeris denticulate ×Crepidiastrum Keiskeanum, Crepidiastrum platyphyllum×Lactuca squarrosa. The eliminated chromosomes seem to be the paternal ones.
6. Also in the natural hybrid, these fragmentation and elimination of the chromosomes are observed.
7. Some natural hybrid populations consist of old hybrids with the fertilities of more than 90%, and other populations consist of recent hybrids with the fertility of some 20%.
