1'1. Once the floristic composition of any plant community is given, the problem of its pattern and structure involves consideration of two inter-related aspects, namely, distribution (frequency of occurrence and mode of dispersion (Clapham, 1936; Blackman, 1942) of the constituent species, their spatial relations, and the variation of these with time) and success (towards the concept of which variables such as age, height, yield, etc., contribute). The effects of habitat variation, in space and in time, on success must then be considered in order that conclusions may be drawn regarding succession within the community, and the internal cycle of change within each of its constituent phases (Watt, 1947a). It is with this last that we are primarily concerned here. 1-2. The investigations have been carried out in woodlands on the shallow, sandy soils of the Millstone Grit and Coal Measures in the neighbourhood of Sheffield, with a ground flora consisting essentially of a mosaic of Holcus mollis L. and Deschampsia flexuosa (L.) Trin. We wished to dissociate the effects of edaphic variation on the success of Holcus mollis and Deschampsiaflexuosa from the complex of factors, environmental and internal, influencing this success. To achieve this it was desirable to maintain, as far as possible, some degree of uniformity in the effects of such other factors. In ecological experiments of the type considered, such uniformity, at least of the other environmental effects, may be achieved qualitatively by observation. But uniformity in the effects of such internal factors as age, genetic constitution, etc., of the respective Holcus and Deschampsia colonies cannot be so achieved. Such colonies, however, reproduce primarily by vegetative means under woodland conditions, and hence genetic variation is probably of minor effect in producing 'within-colony variation', its importance probably lying more in the 'between-colony' differences. Age, on the other hand, has probably both a betweencolony effect, due to different dates of initial colonization of the various areas, and a within-colony effect, due to the occurrence of an internal cycle of development similar to that shown by Watt (1947 b) to exist in Breckland Pteridium colonies, and by Ovington (1947) in Holcus colonies in a Pennine dough woodland. In the preliminary investigations below, the effects of such 'internal' factors are disregarded, their variation contributing to experimental error. 1-3. An investigation such as the above suggests a method of obtaining information about the physiological requirements of species in natural plant communities. Thus, assuming that in a plant community the factors found to influence the success and incidence of any species to the greatest extent are also those which are of greatest J. Ecol. 40 26