Protonemata of the moss Ceratodon purpureus cultured in white light were transferred to darkness for 3 days and then used for phototropic experiments. Irradiation of the apical region of vertically position protonemata with small beams (0.2 mm) of red light induced a growth response towards the irradiated side (positive phototropism). The phototropic response showed irradiance dependence. The effect of red light was completely reversed by far-red light following red light irradiations, demonstrating that phytochrome was the photoreceptor pigment. Far-red light or UV-blue light had no influence on either bulging or phototropism. Experiments with linearly polarized red or far-red light showed a different dichroic distribution of phytochrome in its different forms, the red-absorbing form, P, and the far-red-absorbing form, Pfr. Red light with a vibration plane parallel to the long axis of the filaments was most effective. The effectiveness of far-red light was expressed best when its vibration plane was 90" to the electrical vector of the inductive red light.

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SUMMARY The morphological nature of the tissue regenerating from, the sporogonium of a moss depends upon the physiological age of the regenerating zone. After the sporogonium has attained a certain stage of ageing, only protonema is formed, irrespective of the zone. If the regeneration is carried out in the tip region of very young sporogonia, still largely embryonic, factors influencing differentiation pass into the regnerated tissue. Tissue regenerated from the extreme tip retains embryonic features and consists of undifferentiated, apolar, callus cells. In the subsequent zone, which later on gives rise to the apophysis, other factors are present, and the regenerating tissue gives rise to new setae. The factors responsible for controlling the differentiation of tissue regenerating from the meristematic zones cannot be detected in the subsequent extension zone by regeneration experiments. Some influence persists, however, in the intermediate transition zone, and it is transmitted to and propagated in protonema arising in this region. Such protonema in certain conditions give rise to sporogonia apogamously. Although callus cultures give rise to protonema in darkness, a specific differentiation factor is present since callus is invariably reformed on illumination. All the forms of regenerated tissue (callus, that giving rise to setae, and protonema giving rise to sporophytes) can be maintained in culture. In a strain of hybrid origin, the stability of the seta form in culture is so high that it is comparable with the independent sporophytic regeneration of a higher archegoniate plant. The callus and seta forms regenerated from the sporophyte have marked heterotrophic tendencies. In addition gametophyte and sporophyte produce secondary plant products differing qualitatively as well as quantitatively.

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