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Germination of the cactus species Ferocactus peninsulae and Stenocereus gummosus, and their water requirements and germination responses to discontinuous hydration were studied. Both species germinate slowly and have low final germination percentages. Under laboratory conditions, a gradual decrease in substrate water content simulated a decrease in available soil water with time, a common phenomenon because of water evaporation after a rain in the desert. The experiments compared germination at continuous and discontinuous hydration under various regimes of gradual decrease in available water and with unlimited available water. In a desert environment, discontinuous hydration appears to be a facultative requirement for seed germination because: a) under conditions of gradual decreases in amount of water, continuously-hydrated seeds germinated no higher than on average 15%, b) these species germinated well under optimal conditions of unlimited available water (55 to 66%), which is rare in natural desert environments; and c) seeds subjected to discontinuous hydration had much higher final germination percentages than those hydrated continuously. These two cactus species have "low risk strategy" because: 1) to germinate they require a few days for inhibition and a threshold level of substrate moisture content when subjected to hydration, and 2) they are resistant to long dehydration periods and germinate rapidly after a subsequent rain.