Studies have been made of the relative frequency and distribution in parts of Britain of the chromosome races of Holcus mollis (2n= 28, 35, 42, 49). These show that the penta-ploid is the most common race and that the distribution of the races as a whole is a reflection of varying degrees of adaptability to moisture content of the soil.
Chromosome studies at meiosis and mitosis show that the tetraploid is an allopolyploids and that the hexaploid is derived from it. The penta-ploid appears to be derived from a hybrid of H. lanatus (2n=14) and H. mollis (2n= 28).
The hybrid nature of the penta-ploid is confirmed by the discovery of natural triploid hybrids (2n= 21) from which it is presumed to have been derived by backcrossing to the tetraploid parent.
With the exception of the tetraploid, the races of H. mollis reproduce almost exclusively by vegetative means. The tetraploid is fertile but is also likely to reproduce mainly by rhizomes.
The success of the penta-ploid is attributed to its hybrid vigour in both growth and adaptability to soil moisture. In addition its spread is favoured by the fact that its tetraploid parent does not offer much seed competition in the colonization of new areas. There is no competition from the H. lanatus parent, which has different ecological requirements.
It is considered feasible that the vigorous penta-ploid could eventually supersede the other races of H. mollis.