PHYTOCHROME‐MEDIATED PHOTOTROPISM AND DIFFERENT DICHROIC ORIENTATION OF Pr AND Pfr IN PROTONEMATA OF THE FERN ADIANTUM CAPILLUS‐VENERIS L.

Single‐celled protonemata of Adiantum capillus‐veneris were cultured under continuous red light for 6 days and then in the dark for 15 h. Brief local exposure of a flank (5 times 20 /mi) of the subapical region of a protonema to a microbeam of red light effectively induced a phototropic response toward the irradiated side. The degree of the response was dependent upon the fluence of the red light. Red/far‐red reversibility was typically observed in this photoreaction, showing that phytochrome was the photo‐receptive pigment. When the flank was irradiated with a microbeam of linearly polarized red and far‐red light, red light with an electrical vector parallel to the cell surface was most effective. However, the far‐red light effect was most prominent when its electrical vector was normal to the cell surface. These polarized light effects indicate the different dichroic orientation of Pr (red‐light‐absorbing form of phytochrome) and Pr (far‐red‐light‐absorbing form of phytochrome) at the cell flank.