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Cleistogamy is more common in grasses than in any other angiosperm family. Both self-fertilized cleistogamous (CL) spikelets and open-pollinated chasmogamous (CH) spikelets are typically produced. Relative allocation to CL and CH varies among species and populations, and is influenced by ontogeny and environment. The balance between reproductive modes can be expressed as a CH/CL ratio. This ratio is very plastic, and stressful conditions can result in values Amphicarpum purshii, an annual with subterranean CL spikelets, CH/CL declined as density increased because CH decreased more than CL as size was reduced by intraspecific competition. In the shade-tolerant annual Microstegium vimineum, CH/CL was lowest in large greenhouse-grown plants in an unlimited, sunny environment, but was highest in small plants from a shady forest interior; tiller vegetative mass often showed a negative allometric relation to CH and CL allocation. In the perennial Dichanthelium clandestinum, CH and CL allocation varied among populations, but there was no consistent effect of light on CH/CL. The phenology of reproduction strongly affects CH/CL. In Danthonia spicata, CH/CL was high early in the season as CH flowering commenced, but dropped quickly as axillary CL spikelets matured; A. purshii showed the opposite pattern because CL reproduction occurred first. The assumption that cleistogamy simply provides reproductive assurance should be reevaluated in light of new information on phenology and allometry. Changes in the balance between CH and CL caused by environmental factors may be indirect effects of size. Evolutionary models that do not explore the plasticity and allometry of CH and CL reproduction may not be useful in predicting the myriad patterns in nature.