Big-tooth aspen displays heterophylly on twigs of saplings and on upper, vigorously growing twigs of larger trees. Early leaves, those present in overwintering buds which expand in spring, differ from late leaves, those that are mostly initiated and developed during the late spring and summer of the same year. The two sets of leaves differ in size, shape, number of teeth, number of marginal resin glands, and number and size of basilaminar extrafloral nectaries. Secretory structures are generally more prominent and active on late leaves. Marginal resin glands, resin-secreting stipules, and basilaminar nectaries of both early and late leaves are similar anatomically, having a secretory epidermis of cytoplasmically dense palisade-like cells separated from vascular bundle endings by several layers of isodiametric parenchyma. These secretory structures, and the dense mat of trichomes which cover the leaf until it fully expands, may help to protect the young leaves from insect damage. The basilaminar nectaries seem especially effective because they attract ants that probably discourage visits by other insects.