SummaryBased on empirical evidence from the literature we propose that, in nature, phenotypic plasticity in plants is usually expressed at a subindividual level. While reaction norms (i.e. the type and the degree of plant responses to environmental variation) are a property of genotypes, they are expressed at the level of modular subunits in most plants. We thus contend that phenotypic plasticity is not a whole-plant response, but a property of individual meristems, leaves, branches and roots, triggered by local environmental conditions. Communication and behavioural integration of interconnected modules can change the local responses in different ways: it may enhance or diminish local plastic effects, thereby increasing or decreasing the differences between integrated modules exposed to different conditions. Modular integration can also induce qualitatively different responses, which are not expressed if all modules experience the same conditions. We propose that the response of a plant to its environment is the sum of all modular responses to their local conditions plus all interaction effects that are due to integration. The local response rules to environmental variation, and the modular interaction rules may be seen as evolving traits targeted by natural selection. Following this notion, whole-plant reaction norms are an integrative by-product of modular plasticity, which has far-reaching methodological, ecological and evolutionary implications.

It is widely reported that plants emit volatile compounds when they are attacked by herbivorous insects, which may be used by parasitoids and predators to locate their host or prey. The study of herbivore-induced plant volatiles and their role in mediating interactions between plants, herbivores and their natural enemies have been primarily based on aboveground systems, generally ignoring the potential interactions between above and belowground infochemical-and food webs. This study examines whether herbivory by Delia radicum feeding on roots of Brassica nigra (black mustard) affects the behaviour of Cotesia glomerata , a parasitoid of the leaf herbivore Pieris brassicae , mediated by changes in plant volatiles. In a semi-field experiment with root-damaged and root-undamaged plants C. glomerata prefers to oviposit in hosts feeding on root-undamaged plants. In addition, in a flight-cage experiment the parasitoid also prefers to search for hosts on plants without root herbivores. Plants exposed to root herbivory were shown to emit a volatile blend characterized by high levels of specific sulphur volatile compounds, which are reported to be highly toxic for insects, combined with low levels of several compounds, i.e. beta-farnesene, reported to act as attractants for herbivorous and carnivorous insects. Our results provide evidence that the foraging behaviour of a parasitoid of an aboveground herbivore can be influenced by belowground herbivores through changes in the plant volatile blend. Such indirect interactions may have profound consequences for the evolution of host selection behaviour in parasitoids, and may play an important role in the structuring and functioning of communities.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Ecology.Summary 1 In natural habitats, essential resources for plant growth are heterogeneously distributed in space, resulting in environmental patchiness with favourable and less favourable microhabitats. Clonal plants may benefit from their ability to share resources between connected ramets experiencing contrasting levels of resource supply. This hypothesis was tested with clones of Potentilla reptans and P. anserina, consisting of a mother rosette and a number of daughters on an attached stolon, grown under homogeneously high and low light conditions, as well as in spatially heterogeneous light environments in which either mother or daughters were shaded. Biomass data were used to carry out an analysis of the costs and benefits of clonal integration. 2 Both species showed high benefits of integration. In P. reptans, connections to unshaded clone-parts enhanced biomass production of the shaded part. In contrast to the prediction that clonal integration would result in costs to unshaded clone-parts connected to shaded parts, unshaded ramets of both species developed higher instead of lower dry weights when connected to a shaded part. Thus, unshaded ramets actually benefitted from integration. 3 It is hypothesized that these unexpected benefits of integration were due to mutual support of connected clone-parts growing in shaded and unshaded patches. The high evaporative demands of the unshaded clone-parts probably resulted in water being translocated from shaded to unshaded ramets while, at the same time, carbohydrates were translocated from unshaded to shaded ramets. 4 We discuss implications of this suggested bidirectional transport of two different resources for the ecological significance of physiological integration in clonal plants and for the notion of habitat heterogeneity.

Environmental heterogeneity has received wide attention in clonal plant research over the last decade. Most studies have focussed on the effects of spatial variation in environmental conditions on the performance of ramets and genets, while the effects of temporal heterogeneity have only occasionally been studied. As a consequence, our current knowledge about functional responses of clonal plants to habitat patchiness is biased towards spatial aspects of environmental heterogeneity. Nevertheless, temporal changes in biotic and abiotic conditions do occur in most natural habitats, and they are very likely to affect plant growth and performance, and to create positive selection pressures on traits that can buffer plants against unfavorable consequences of this variability. Storage of resources is a widespread phenomenon in clonal plant species. Typical clonal structures such as stolons, rhizomes and hibernacles serve as storage organs. However, the ecologic significance of storage in clonal plant structures remains partly unclear. We suggest that the lack of understanding with respect to resource storage in clonal plants be related to our poor knowledge of ecologic implications of temporal habitat heterogeneity in natural environments. Resource storage can be understood as a safety measure against temporal changes in the growing conditions of plants. This paper summarizes existing information about the ecologic relevance of storage in clonal plants and it tries to develop a framework for further investigation of resource storage as a strategy to enhance the performance of clonal plants in temporally variable environments.

Plant ecologists have spent considerable effort investigating the physiological mechanisms and ecological consequences of clonal growth in plants. One line of research is concerned with the response of clonal plants to environmental heterogeneity. Several concepts and hypotheses have been formulated so far, suggesting that intra-clonal resource translocation, morphological plasticity on different organizational levels (e.g. leaves, ramets, fragments), and other features of clonal plants may represent potentially adaptive traits enabling stoloniferous and rhizomatous species to cope better with habitat patchiness. Although each of these concepts contributes substantially to our understanding of the ecology of clonal species, it is difficult to combine them into a consistent theoretical framework. This apparent lack of conceptual coherence seems partly be caused by an uncritical use of the term 'habitat heterogeneity'. Researchers have not always acknowledged the fact that 'heterogeneity' may refer to a number of fundamentally different aspects of environmental variability (i.e. scale, contrast, predictability, temporal vs. spatial heterogeneity), and that each of these aspects may, on one hand, allow for the evolution of specific plant responses to heterogeneity and, on the other, severely constrain the viability of potentially adaptive traits. Since adaptive responses are operational only in a narrow range of conditions (delimited by external environmental conditions and constraints internal to plants) it seems imperative to clearly define the context and the limits within which concepts regarding clonal plants' responses to heterogeneity are valid. In this paper an attempt is made to review a number of these concepts and to try and identify the necessary conditions for them to be operational. Special attention is paid (1) to different aspects of environmental heterogeneity and how they may affect clonal plants, and (2) to possible constraints (e.g. sectoriality, perception of environmental signals, morphological plasticity) on plant responses to patchiness.