THE NATURE AND RANGE OF VARIATION IN THE FLORAL SYMMETRY OF POTENTILLA ERECTA (L.) HAMPE

Abstract

More than fifty years ago I read to the Linnean Society a paper on this subject, prepared in collaboration with Miss Elizabeth Dale. I was somewhat dissatisfied with it at the time because of the number of 'loose threads' involved in the complex problems of variation in which I was then interested, and I did not see how to follow them up; so the paper was never submitted for publication. In the middle nineties of the last century the subject of variation was very prominent in the minds of biologists. Bateson's thick volume, Materials for the Study of Variation, had recently been published, and controversy raged between those who thought that 'continuous' variation was the main, if not the only, material on which natural selection worked and those who stressed the importance of 'discontinuous' variation in evolution. It was, of course, before Johannsen's work on the non-inheritance of 'continuous' (fluctuating) variation, and before the rediscovery of Mendel's work and the revolution which it brought into this whole field. My attention had been attracted by the constant occurrence of a small percentage of pentamerous flowers in the common tormentil, Potentilla erecta (then usually known as Potentilla Tormentilla), whose normal 4-petalled flowers are familiar to everyone, since the tormentil is one of the most widely distributed and commonest plants in rough grazings, heathlands and moorlands on the lighter non-calcareous soils throughout the country. Influenced by the notion that particular environmental conditions, such for instance as soil differences, water supply, local climate, or aspect, might affect this kind of variation Miss Dale and I collected and examined a large number of flowers from various sites in different parts of the country, but got no clear indications of any such effects. I did, however, conclude that the plant produces more pentamerous flowers at the outset of its flowering season than later in the summer. Thus I observed IO-I2z% of pentamerous flowers borne by luxuriant plants with long shoots which had grown up through gorse bushes and just opened their first flowers in June, at a time when the plants growing in grazed turf, and thus fully exposed to light from the first, had been flowering for some weeks. It is possible that the large proportion of pentamerous flowers (about four times the normal percentage) on these plants growing among gorse was connected with their luxuriance, but the correlation with early production is confirmed by the larger proportion of pentamerous flowers observable in early May, when fully exposed tormentils begin to flower, than later in the season. Further confirmation was obtained by observing the symmetry of successively opening flowers in pot cultures. Unfortunately, no systematic observations were made on the distribution on individual plants or on the seasonal distribution of variation in the symmetry of the flowers. It may be conjectured that pentamerous flowers are largely the terminal flowers of the main flowering axes, as