PHOTOTROPISM OF SELAGINELLA: THE DIFFERENTIAL RESPONSE TO LIGHT

Selaginella kraussiana (Kunze) A. Braun has a plagiotropic dorsiventral stem with two rows of small leaves on the dorsal surface and two rows of large leaves inserted laterally. Stem tips exhibit a differential phototropic response. When stem tips are placed in their normal horizontal orientation and the dorsal surfaces are illumianted, the tips bend only 20” below the horizon and away from light. Stem curvature is limited to a zone 450 μm long located 1.5 mm behind the shoot apex. The dorsal cortical cells within this zone of curvature are about 1.44 times longer than the ventral cortical cells. Illumination of the ventral surface of the stem tips elicits a strong phototropic response. The stems bend from 123–158° below the horizon and toward light, and the zone of curvature increases in length to 10 mm of the explant. The curvature is large enough so that the previously shaded dorsal leaves of the stem tips become redirected toward the light. This phototropic response is promoted by white and blue light, whereas red or far-red light has no effect. When stem tips are cultured in total darkness, the length of the zone of curvature is 8.0 mm but the stems bend only 50–67°. Treatment of the small dorsal leaves with phenylacetic acid inhibits phototropic curvature, and the phototropic response is unaffected by gravity.