The growth Á /differentiation balance (GDB) hypothesis (as elaborated by Herms and Mattson) provides a framework for examining the impact of a resource gradient on the constant tradeoff between growth and differentiation in cells and tissues of plants, in particular with the consequences for plant defense. The GDB hypothesis, which is the most mature of the hypotheses of plant defensive levels, has not been tested directly. Examination of the requirements for a rigorous test indicates that, like the other hypotheses of plant defense, it cannot be tested directly. Furthermore, rigorously testing the primary derivative hypotheses, while possible, would require considerable methodological effort, on a scale not previously attempted for tests of plant defense, which is likely to discourage researchers, and understandably so, even though the GDB hypothesis warrants methodical investigation due to its potential explanatory power. Although farther removed from the abstract model (i.e. the GDB framework), other derivative hypotheses can be tested, but doing so will require thoughtful consideration and acknowledgement of that. Study of a few carefully chosen systems (i.e. plant species) may provide considerable insight and potentially useful refinement of the GDB framework.The growth Á/differentiation balance (GDB) hypothesis predicts how plants allocate between differentiationrelated processes and growth-related processes in different environmental conditions (Loomis 1932(Loomis , 1953. Growth is the production of roots, stems and leaves, or any process that requires substantial cell division and elongation. In contrast, differentiation is the enhancement of the structure or function of existing cells. Some examples of differentiation-related processes are secondary metabolism, trichome production and thickening of leaf cuticle, all of which can limit herbivory (Herms and Mattson 1992). Allocation to differentiation includes cost of enzymes, transport and storage structures involved in defense. Growth and secondary metabolism can compete for available photosynthate (Wadleigh et al. 1946, Veihmeyer and Hendrickson 1961, Mooney and Chu 1974, and so there is a trade-off for carbon allocation (Lorio 1986).The GDB hypothesis predicts that any factor that slows growth more than it slows photosynthesis can increase the internal resources available for allocation to differentiation (Loomis 1932(Loomis , 1953. For instance, growth is slowed by limitation of nutrients and water, whereas photosynthesis is less sensitive to such shortages (reviewed by Herms and Mattson 1992). Consequently, carbohydrates accumulate beyond growth demands and, thus, may be converted to secondary metabolites, with little additional cost to the plant, at least in terms of soil nutrients. Herms and Mattson's (1992) expansion of the GDB hypothesis provides an explanation for phenotypic variation in concentrations of secondary metabolites in plants and for predicting the consequences of that variation on plant Á/invertebrate herbivore interactions. Furtherm...