'Every being, which during its natural life time produces several eggs or seeds, must suffer destruction during some period of its life, and during some season or occasional year, otherwise, on the principle of geometrical increase, its numbers would quickly become so inordinately great that no country could support the product' (Darwin 1859). It is the task of the demographer to describe and attempt to explain numbers and changes in numbers of organisms and most of our present knowledge of the biology of populations has been based upon studies of animals. Very few papers provide even elementary actuarial data about plant populations. The behaviour of a population is dominated by the births, deaths and migrations of its individuals. In plants, birth, that is to say seed production and vegetative multiplication, has been the subject of extensive anecdotal biology and the elegant beginnings of a systematic treatment by Salisbury (1942). Migration has been examined primarily in anecdotal examples and mortality has been particularly neglected. Mortality in some crop and weed populations has been studied in field plot experiments (Harper & Gajic 1961; Harper & McNaughton 1962) and formal analyses of such simple populations have been developed and reviewed by Yoda et al. (1963) and White & Harper (1970). However, mortality data from natural populations have rarely been recorded, perhaps partly because it is more difficult to define 'the individual' in plants than it is in animals. The estimation of annual weed populations and their reproductive potential in a sequence of years can provide the necessary elements to show how plant numbers change from year to year (e.g. Avena fatuat, Selman 1970). For other annuals, which are not privileged by agronomic interest, data on population regulation are almost non-existent. A remarkable exception is the study by Sharitz (1970) of two members of a seral community (Minuartia unifora and Sedum smallii) in south-eastern United States, where a detailed study of plant numbers was complemented by observations of environmental factors and experimental work on the effects of physical and biotic factors on the regulation of the population. Although many forestry investigations contain casual information on the mortality of individuals, and, despite the advantage that the age of many trees can easily be determined, no full demographic study appears to have been made on woody perennials. The