The pioneer vegetation of exposed muds and its biological features

Abstract

The characteristic pioneer flora of exposed mud in Britain, here considered, is held to comprise about fifty species, most of which are local in their occurrence, often conspicuously intermittent, and very variable in their abundance. The majority are annuals whilst others, potentially perennial, may behave as annuals. Their provenance is prevailingly Southern or Continental. The fruiting periods exhibit a marked late-summer or autumnal bias corresponding with the season of greatest evaporation and frequency of suitable habitat conditions. Most of the ‘mud-species’ have greatly diminished in their frequency, the average loss being probably about 40% of their former locations. The importance of recording their biological characteristics in Britain, before they become extinct, is emphasized. The propagules are shown to be generally small, or very small, with transport by adhesion to the feet and feathers of water-birds as probably the most frequent means of more distant dispersal, although internal carriage may be effective for some species over long distances. Estimates respecting the average and observed maximum outputs for 36 species are furnished and, as with other ‘intermittently available habitats’, these are shown to be very large especially in relation to the size of most of the plants. The mode of germination is illustrated by data from experimental sowings, of freshly ripened propagules, of 38 of these species. The germination behaviour, it is shown, may vary conspicuously as between the different species, sometimes even between individuals of the same taxon, but there is a prevailing tendency, especially marked with some species, towards quasi-simultaneous germination of the seeds or fruits maturing at the same time. Germination experiments, involving large numbers of seeds or fruits from different individuals, have shown that while, frequently, there may be no germination in darkness, the propagules from certain individuals of the same species may yield a small or moderate percentage in the dark. The response to light is probably a matter of degree rather than of kind. Whilst the average behaviour, in this and other respects, is of great significance it is emphasized that the diversity of biological characteristics, which experimental cultures reveal, in the offspring of different individuals (termed ‘polybiosis’) may be of great importance for the exploitation of abnormal circumstances and perhaps even for survival. Most ‘mud-species’ have very small propagules and either exhibit no germination in darkness or a very low percentage. The small number of species with relatively large propagules yield appreciable germinations in the dark. These ‘mud-species’ exhibit, in varying degrees, significant specialization for the colonization of bare mud and are intolerant of competition. Cogent evidence is furnished that emphasizes the importance, for experiments on wild species, of utilizing propagules derived from a single individual to permit of valid conclusions as to the effect of contrasting conditions.