Tetraploid and hexaploid chromosome races of Phalaris arundinacea L.

Chromosome counts of a wide range of Phalaris arundinacea introductions have revealed the existence of two chromosome races, one tetraploid (2n = 28) and the other hexaploid (2n = 42). The majority of the introductions, including those from the Soviet Union, northern and central Europe, and North America, belong to the tetraploid race, whereas those from Portugal, Spain, Oregon, U.S.A., and two Botanic Garden collections in central Europe belong to the hexaploid race. Both races have regular meiosis and a stable chromosome number, and are fully fertile. The hexaploid hybridizes freely with the tetraploid Ph. Arundinacea and also with Ph. Tuberosa, but is effectively isolated from both owing to the sterility of the hybrids. The hexaploid differs from the tetraploid in a number of characteristics, and in particular in its growth rhythm. Under Australian conditions it has a longer growing season, and is more productive in the autumn and winter. In this and in other respects the hexaploid is intermediate between the tetraploid form of Ph. Arundinacea and Ph. Tuberosa, and is considered to have arisen through hybridization and subsequent introgression between these two species along the junction of their respective distributions in southern Europe.