An Investigation of Shoot Interactions in Mercurialis Perennis L., A Rhizomatous Perennial Herb

Most investigations into the development of plant populations have used annual species grown either under partly controlled field conditions or in glasshouses. Such an approach may enable the effect of single variables upon the population to be investigated and overcomes the problems of defining the individual when vegetative reproduction occurs. However, it is well known that extrapolation to field conditions of results from such experimental populations can be very misleading. In particular, in the case of vegetatively reproducing perennial species, the identity of the basic population unit is obscure when adult plants and their rametst are growing together. Structures such as shoots or tillers which are interconnected may not function independently. Integration has been demonstrated, for example, between tillers of plants of Lolium multiflorumt even though each appears to be capable of independent status in terms of carbon economy (Marshall & Sagar 1968). The same has been shown for developing plants of wheat (Quinlan & Sagar 1962). Sonneveld (1962) and Struik (1965) have outlined schemes of translocation and redistribution of assimilates which imply integration and interdependence between different parts of plants at different times during their growth. Despite this, the tiller, grass tuft or herb shoot has often been arbitrarily chosen as the basic unit in plant ecological studies, particularly those concerned with distribution of particular species within stands of vegetation (e.g. Whitford 1949). Clearly, direct analogy is not possible between these units and individual plants in experimental populations. If tillers, tufts or shoots are not physiologically independent entities, it is important to determine the role they play in the development of a population. It is the purpose of this study to determine the extent to which shoots of the woodland perennial herb Mercurialis perennis growing in natural stands are comparable with discrete units in populations. At maturity, a dense, pure stand of M. perennis forms a virtually closed herb canopy with few gaps through which light can penetrate directly to the ground. This complete ground cover develops early in the growing season and is important in excluding competitor plants from areas dominated by M. perennis (Wardle 1959; Pigott & Martin 1964; Wilson 1968). In addition, M. perennis does not exhibit clonal senescence over