In experiments with plants grown in pots, vesicular—arbuscular mycorrhizal increased seedling growth of 23 to 28 species from a lowland tropical rain forest region. Mycorrhizae improved survival in six species and cotyledon retention in five species. Mycorrhizae also increased the size of bacterial nodules and the proportion of nodulated plants among three leguminous species. Growth of seedlings lacking mycorrhizae slowed greatly or ceased after attainment of sizes correlated with average seed dry mass. Removing cotyledons from individuals of two species that are dependent on mycorrhizae, however, did not increase their response to inoculation. Thus, seed reserves are important for mycorrhiza formation as well as for seedling growth before infection. Large seeds are advantageous to plants that depend on vesicular—arbuscular mycorrhizae because they provide mineral reserves upon which the seedling can draw while awaiting infection. Seedlings of some species could not grow without mycorrhizae, but inoculation did not affect the growth of other species. Species that are least dependent on mycorrhizae have light seeds and colonize disturbed habitats.