Adaptation to Habitat in a Group of Annual Plants

Clapham (1946) has emphasized that the persistence of a species at a given site is dependent upon the interaction of plant and environment throughout the whole of the life-cycle, at the stages of germination, establishment, vegetative growth, flowering and the setting of viable seeds. He points out that the elucidation of the way in which a species thus interacts with its environment can most profitably be investigated by experimental methods. This work represents an attempt to investigate experimentally, using relatively simple apparatus, the basis of adaptation to a rather extreme habitat in a well-defined group of annuals on the Derbyshire limestone. As annuals, their survival in any locality depends upon the successful accomplishment every year of all the above stages whatever the seasonal and yearly variations of climate. Despite this variation the life-cycles are remarkably similar and constant: germination invariably takes place in the early autumn, usually in midor late September; vegetative growth then proceeds until some time in December and with the onset of more favourable conditions in the spring, flowering occurs; seeds are shed in May or June but no germination takes place until the following autumn. As will be seen later, this life-cycle fits rather well into the local climatic pattern, but its significance and the way in which it is controlled was not at all obvious. The habitat is very clearly defined as the species are only to be found on shallow soil in open, sunny situations. On the Derbyshire limestone these conditions are provided in abundance on rock ledges, screes, etc., with a more or less southerly aspect where the annuals in question form a distinct community. Sociologically, therefore, the situation is very simple, particularly as competition between individuals or species is almost nonexistent. The following species may be included in the community, varying somewhat in abundance: