Background/Question/Methods Flowering is induced through steps of physiological processes, which in turn constrain the relationship between size and reproduction. Summer annuals need a short-day photoperiod for flowering, and winter annuals a long day. Winter annuals and biennials must be exposed to low temperatures before receiving a long-day photoperiod. Biennials have to be large enough to respond to the photoperiod. They have a threshold plant size for flowering and consequently stay vegetative for more than two years when growth is slow. The threshold and the photoperiodic response are expected to maximize the fitness under a given environment. However, the extent to which this is so is not well documented. As monocarpic species have no perennating structures to leave after reproduction, they transfer as much resources as possible to offsprings (seeds) before dying. Results/Conclusions We studied the life history of a biennial Oenothera glazioviana in a sand dune system. The plant must be at least 10 cm in rosette diameter before flowering (Kachi & Hirose 1983). This threshold size maximized the intrinsic rate of natural increase despite delay in reproduction (Kachi & Hirose 1985). Large threshold size is favored in the environment imposing high mortality and/or low productivity (Hirose & Kachi 1986). With Xanthium canadense, we did a serial study on reproductive allocation of annual plants. Xanthium flowered strictly responding to a short-day photoperiod irrespective of the size (Sugiyama & Hirose 1991). In the natural environment, there were large differences in germination time. Reproductive yield was higher in early germinators, while reproductive effort (the ratio of reproductive to vegetative mass) was higher in late germinators (Shitaka & Hirose 1993). When flowering time was manipulated by controlling photoperiod, both early and late flowering decreased reproductive yield as compared with natural flowering (Shitaka & Hirose 1998). Reproductive effort was larger and the reproductive period was shorter in early, natural and late flowering plants in this order. Elevated CO2 increased the vegetative growth, but did not the reproductive yield (Kinugasa et al. 2003). This was because seed nitrogen concentration was high in Xanthium and the reproductive yield was limited by the availability of nitrogen more than of carbon. A model was constructed to have an overview on plant reproduction under a limited availability of nitrogen (Hirose et al. 2005). We conclude that size at reproduction may be determined so as to maximize the fitness, but that the relationship between size and reproduction changes depending on growth conditions.