In order to investigate the pattern of population differentiation in Agrostis tennis, sixty tillers were collected from each of thirty-three areas mainly in central Wales. These were grown under garden conditions in Aberystwyth. In a subsequent experiment five of these areas were sampled again by tillers, and at the same time sampled by seed. These samples were grown under garden conditions in Bangor.

Numerous differences between these population samples were recorded. The main conclusions are as follows:

(i) The environment is the dominating factor in determining population differentiation.

(ii) Although the distribution is continuous, distances of about 50 m or less are sufficient to effectively isolate populations from one another.

(iii) Thus the species is able to evolve under the influence of natural selection in response to very local variations in environment.

(iv) Since the environment is a graded patchwork of different conditions, the pattern of differentiation is similar.

(v) It is, therefore, not possible to classify effectively such interspecific variation.

(vi) It is likely that this situation is to be found in most other out breeding continuously distributed plant species occupying a wide range of habitats.