Plant characteristics that determine food quantity and quality to consumers exhibit extensive within-plant heterogeneity, and this heterogeneity is an important influence on the interactions between plants and consumers (herbivores, pathogens, mutualists, soil-dwelling microorganisms). Here we present a functional model – based on plant vascular architecture and local environmental variability – that can be used to predict the patterns of within-plant resource heterogeneity. We argue that heterogeneity is generated largely by sectoriality, the restricted movement of resources along vascular traces within a plant. In essence, the combination of sectoriality and spatial variation in previous damage, nutrient, water, and light availability generates predictable patterns of within-plant heterogeneity in tissue quality. We point out that vascular architecture differs across taxa, growth habit and plant developmental stage, and suggest that certain attributes of the environment maximize the extent of heterogeneity.