There seems to be little barrier to natural hybridization between Agrostis tennis Sibth. and A. stolonifera L. The F1 hybrid is vigorous: but it is relatively sterile.

The analysis of natural mixed populations of these two species from an old grazing meadow by means of a morphological index and by pollen fertilities shows that over a large central area of the meadow, plants indistinguishable from Fl hybrids are very abundant, to the virtual exclusion of the parental species which would be expected in such a habitat. A more elaborate morphological and cytological analysis confirms this, but suggests that there is also a small percentage of plants belonging to F2 and backcross generations.

The success of the hybrid is due to its growth habit, which is better adapted to grazing conditions than that of either parent. Under grazing conditions reproductive sterility is no disadvantage.

The hybrid populations are made up of a large number of different individual F1 hybrid plants, and cannot be due to the spread of one hybrid plant throughout the meadow, but to the repeated formation of the hybrid. Preliminary examination of other habitats suggests that the hybrid is more common than previously suspected.

Discussion from an ecological standpoint suggests that the success of natural sterile hybrids depends on the possession of strong powers of vegetative propagation originally derived from the parental species. For this reason Agrostis tennis stolonifera is particularly successful. There are, however, other contributory factors.

Discussion from a genetical standpoint suggests that since the hybrid is sterile and subsequent generations are uncommon, the two species are likely to remain quite distinct, but that gene flow between them cannot be ruled out without further investigation.