Seed heteromorphism related to chasmogamous (CH) and cleistogamous (CL) seeds can have different ecological significance in species with different functional characteristics, competitive ability and palatability. Punagrass [<i>Achnatherum brachychaetum </i> (Godr.) Barkworth], an invasive, perennial grass is common in grasslands of Argentina and it is an aggressive weed in other parts of the world. <i>Flechilla grande</i> [<i>Nassella clarazii</i> (Ball) Barkworth] is a palatable perennial grass associated with the dominant "climax" vegetation in grasslands of Argentina. Seeds of the two grasses were collected from grasslands of Argentina, and growth chamber and greenhouse experiments were conducted to determine: 1) germination, dormancy breaking, and mass of CH and CL seeds of the two species 2) effects of contrasting range condition on germination and seed mass of punagrass, 3) the relative fitness of plants from CH and CL seeds, 4) if contrasting range condition affect fitness of CH plants of punagrass, and 5) the effect of maternal nutrient environments on CH and CL seeds and on fitness in the two species. Small CH seeds of high dispersal potential were less dormant than large CL seeds of low dispersal potential in punagrass. CH and CL seeds of flechilla grande had similar mass, germination, and response to dehulling. CL seed size and CL seed production of punagrass increased with good range condition. Increasing the maternal, nutrient environment enhanced germination of CH seeds, CL seed size, growth rate, development, biomass and seed production more in punagrass than flechilla grande. Under low nutrient conditions, flechilla grande produced a few large CH seeds. CH progeny of punagrass grew fast and developed rapidly as compared to CL progeny, which in turn produced many CH seeds. CH and CL seeds of flechilla grande had similar contribution to the fitness of adult plants. In both species, the ecological significance of having seed heteromorphism is that sibling competition is probably reduced by having more diverse offspring. Heavy grazing of competitive species such as flechilla grande may favours species as punagrass with many small CH seeds, high potential for colonization as well as large CL seeds for persistence in the seed bank and seedling competition.