Pheromonal interactions between cordate gametophytes of the lady fern, A. filix-femina, were assayed using a protocol typically used for detecting water-soluble pheromones such as antheridiogen. Three week-old, cordate gametophytes were transferred from multispore cultures grown on nutrient agar to agar containing extracts from a previous generation of gametophytes (treatment) and to fresh nutrient agar (control). Three weeks after transfer, fifty gametophytes were examined from treatment and control plates. Each gametophyte was measured for size (area) and shape (circularity) and scored for number of antheridia and archegonia. Treatment gametophytes were significantly smaller, less circular, had fewer archegonia, and possessed antheridia more often than control gametophytes, a pattern consistent with known antheridiogen effects on gametophytes of transitional morphology and sensitivity. The experiment was repeated using gametophytes that were six weeks old at time of transfer to treatment and control plates. Treatment gametophytes in the second experiment did not differ significantly in size (area) or length from control gametophytes; however treatment gametophytes were more circular and possessed greater widths and length : width ratios, deeper notches, and fewer archegonia. We present a model in which one or more phytochemicals released by cordate gametophytes increase rates of anticlinal division in the apical meristem. The possibilities that the substances involved are phytohormones involved in the development of a notch meristem and cordate morphology in the source gametophyte, and that antheridiogen may be involved, are explored.