AbstractPlasticity in growth, reproductive energy allocation (RA), reproductive output (PN), propagule weight (Pw), fecundity, and relative cost of producing single propagules (RA) and their interlinked properties were critically investigated in two grass species,Oryza sativaL. ev.AkihikariandCoix ma‐yuenRoman., grown under varying densities and soil nitrogen levels.In these two species, exceedingly marked plastic but parallel responses were detected in all significant yield and reproductive traits under changing growing conditions. The results obtained also corresponded very well with those forCoix ma‐yuen, Helianthus annuusandGlycine maxreported in our previous works in this series (Kawano and Hayashi, 1977; Kawano and Nagai, 1986; Nagai and Kawano, 1986).InOryza sativa, plants cultivated at higher densities exhibited proportionately lower individual biomasses, lower seed outputs, and smaller grain size in response to increasing density, although RA remained more or less constant throughout all five different densities, and fecundity showed an abrupt decrease in the overcrowded cohort beyond 400 plants/m2. In this experiment,Coix ma‐yuenwas grown only under conditions of extraordinarily overcrowded density (over ca. 4,500 plants/m2) with extremely broad nitrogen fertilizer levels from 20 N (20 kg ammonium sulfate/10 ARE) to 100 N (100 kg/10 ARE). This species likewise demonstrated a remarkably sharp responsive plasticity in significant reproductive parameters with increasing N‐levels, just as was found with changing density (Kawano and Hayashi, 1977).One common significant conclusion obtained from the present series of studies was that sexual propagule production under limited resource availability—for example, a sharp decline in solar radiation and soil nutrients, due to strong interference in higher density plots—is exceedingly costly, implying that there occurs a significant change and variance in this important fitness component under changing environmental conditions. This was most conspicuously demonstrated by a very sharp increase in relative energy partitioning to a single propagule in response to increased density and decreasing nitrogen levels, the relative energy cost to a single seed (RA) increasing from 1.00 to 31.02, and from 1.00 to 10.31 in both paddy field and pot populations ofOryza sativacv.Akihikari, respectively, and from 1.00 to 4.23 in field populations ofCoix ma‐yuen.The adaptive significance of induced changes in several reproductive traits under changing environmental conditions was also discussed in the light of all the evidence av