Cheplick, G. P. (Department of Biology, College of Staten Island, City University of New York, Staten Island, NY 10314). Biomass partitioning and reproductive allocation in the invasive, cleistogamous grass Microstegium vimineum: Influence of the light environment. J. Torrey Bot. Soc. 132: 214–224. 2005.—Biomass partitioning and allocation to chasmogamous (CH) and cleistogamous (CL) reproduction were investigated in the invasive, summer annual Microstegium vimineum in relation to light conditions. This shade-tolerant grass weed of Asian origin forms dense populations in the understory of moist deciduous forests in eastern North America. In autumn each flowering tiller produces one terminal raceme with CH spikelets and 2 to 7 sheath-enclosed axillary racemes with CL spikelets at the uppermost nodes. In a greenhouse, plants were reared from seed families collected from individuals in subpopulations of shady (2 to 8% full sun) or sunny, edge habitats. At maturity, tillers of the shade subpopulation showed greater allocation to leaves, but reduced allocation to CH and CL, compared to those from the sunny subpopulation, suggesting adaptive differentiation to light conditions in the field. Maternal family (within subpopulation) had a significant effect on CH allocation and the mean mass of CH and CL seeds. For mature tillers collected directly from the field habitats, allocation to CH and CL was lowest, and allocation to leaves was greatest, in deep shade. CH and CL allocation averaged 16% and 11%, respectively, in the sunny habitat, but only 6% and 7% in the shady habitat. There was no evidence of a trade-off in allocation to CH vs. CL in field-collected or greenhouse tillers; in fact, there was a significant positive correlation of CH with CL allocation. The ability to grow and allocate limited resources to seed production in CH and CL spikelets under deep shade where other herbs are uncommon, is crucial to the success of M. vimineum as an aggressive colonizer of disturbed forests.