Photocontrol of the orientation of cell division in Adiantum

Summary Two-dimensional prothallia of Adiantum capillus-veneris always expanded in a plane which was at a right angle to any given direction of irradiation with continuous white light. The expansion began with a longitudinal division of the apical cell, in the filamentous protonema, and the orientation of the mitotic cell plate of this first longitudinal division as well as the subsequent divisions was always parallel to the direction of the incident light. When three irradiations with white light, interrupted by periods of darkness, were given, two transverse and one subsequent longitudinal division were induced. When the last two irradiations were given from the same direction, the cell plate of the first longitudinal division in most protonemata was oriented parallel to the direction of light. However, when the direction of light during the third irradiation was at right angle to that during the second, the frequency of the longitudinal division greatly decreased but that of the third transverse division increased. Thus, the orientation of the first longitudinal division appeared to be controlled in some way not only by the irradiation which actually induced the third division but also by that inducing the preceding transverse division, while the direction of light for the first transverse division had little effect on the orientation of the third division.