The four species investigated occur in pioneer habitats but have different distributions. This study of their comparative biology is an attempt to account for these differences.
The literature on the factors determining habitats and geographical distribution of each species is reviewed but this gives little or no evidence as to whether the species are causally related to, or merely associated with these factors in their ecological distribution. Experiments were therefore designed to test the causal relationship and mode of action of the factors, and this first paper describes those dealing with germination.
It appeared from the literature that factors of probable importance in determining the ecological distributions of the species include mineral nutrients and light requirements. The mineral nutritional requirements of Chamaenerion angustifolium seemed to be particularly high. Tussilago farfara appeared to be the most calcicole and Chamaenerion angustifolium the most calcifuge of the species. Epilobium montanum seemed to be the most shade-tolerant. Additional factors appeared likely to be involved at the limits of their geographical distributions; these include, along the southern limits, factors operating through the water relations of the species, and along the northern limits factors operating through the temperature relations and length of season of growth of the species.
The wind-dispersed seeds of the species are of low dry weight. Water culture experiments showed that, with regard to germination, Tussilago farfara is most successful and Chamaenerion angustifolium least successful in very low levels of mineral nutrient supply. The germination of Tussilago farfara in relation to increasing acidity is completely inhibited at a bigher pH (3.5) than it is in the other species (pH 3.0). In T. farfara germination is also restricted to the period mid-April to mid-June by the limited length of its seed viability and season of fruit dispersal. In the other species, though seed shedding is restricted to summer and autumn, the seed remains viable for at least a year and requires light to germinate. In Chamaenerion angustifolium this light requirement disappears as the temperature is raised to 30°C, but it remains in the two Epilobium species. At low temperatures (10°C) maximal percentage germination occurs under lower levels of illumination in E. adenocaulon than it does in Chamaenerion angustifolium, and this probably underlies the much greater percentage germination attained in Epilobium adenocaulon than in Chamaenerion angustifolium when the seeds are shallowly buried (2 mm). Germination in dry conditions also occurs more readily in Epilobium adenocaulon than in Chamaenerion angustifolium. Epilobium montanum is most similar to E. adenocaulon in germination requirements but differs in that its germination is inhibited at 30°C.