Observations on the curvature of Physcomitrium patens (Hedw.) Mitt. and Funaria hygrometrica (Hedw.) caulonemal filaments

Botany, Ahead of Print.
Light-grown, whole gametophytic colonies of Physcomitrium (formerly Physcomitrella) patens (Hedw.) Mitt. exhibit a spiral morphology resulting from the strongly coordinated curvature of the population of peripheral caulonemal filaments. The direction of curvature is predominantly clockwise when cultures are illuminated from above and anticlockwise when illuminated from below. In P. patens, side branch initials (SBIs) emerge from caulonemal subapical cells on the outside of the curve. By contrast, the curvature of caulonemata of Funaria hygrometrica is predominantly anticlockwise when colonies are illuminated from above and clockwise when illuminated from below. In F. hygrometrica, SBIs emerge from caulonemal subapical cells on the inside of the curve. We have discounted a role for gravity in these phenomena and discuss several other possible mechanistic explanations. We also document for the first time thigmotropism of protonemata of P. patens.