Protonemata of Physcomitrium were grown in a sucrose-mineral nutrient, liquid medium. Even in this medium containing organic nutrient, the growth rate of lateral branch, chloronemal filaments showed a light dependence which was linear with log intensity. Intensities necessary to give a constant growth rate (45 μ/1.75 hrs.) were determined at selected wavelengths. The resulting action spectrum paralleled the in-vivo absorption spectrum of a single filament in the red region, showing a major peak at 680 nm. Growth rate and absorption approached zero in the far-red (730 nm). However, significant growth activity occurred at 365–400 nm where absorption was low, and negligible growth was found at 440–500 nm where absorption was high. The action spectrum for the positive, directional photo-orientation of growth was determined by the null-point method in which the effectiveness of each selected wavelength was compared to a 665-nm standard in simultaneous, bilateral irradiation. In contrast to growth, the major peak of phototropic activity was found at 730 nm with significant activity extending to 800 nm. A minor peak was at 680 nm. There was some activity in near ultraviolet but not at longer blue wavelengths. It is concluded that the blue-absorbing system responsible for phototropism in virtually all other groups of plants is inactive or absent in Physcomitrium. Instead growth and orientation seem to be dependent upon an interaction between the photosynthetic and phytochrome systems. Further, the data suggest that the physiological activity of phytochrome in photo-orientation of growth does not derive from a certain amount of P_fr or P_fr/P_r ratio but rather it derives from the simultaneous excitation and consequent cycling of P_r and P_fr.

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