Plants produce a considerable number of structures of one kind, like leaves, flowers, fruits, and seeds, and this reiteration is a quintessential feature of the body plan of higher plants. But since not all structures of the same kind produced by a plant are identical—for instance, different branches on a plant may be male or female, leaf sizes in the sun differ from those in the shade, and fruit sizes can vary depending on patterns of physiological allocation among branches—a single plant genotype generally produces a multiplicity of phenotypic versions of the same organ. Multiplicity in Unity uses this subindividual variation to deepen our understanding of the ecological and evolutionary factors involved in plant-animal interactions. On one hand, phenotypic variation at the subindividual scale has diverse ecological implications for animals that eat plants. On the other hand, by choosing which plants to consume, these animals may constrain or modify plant ontogenetic patterns, developmental stability, and the extent to which feasible phenotypic variants are expressed by individuals. An innovative study of the ecology, morphology, and evolution of modular organisms, Multiplicity in Unity addresses a topic central to our understanding of the diversity of life and the ways in which organisms have coevolved to cope with variable environments.