SUMMARY1The 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.2If the regeneration is carried out in the tip region of very young sporogonia, still largely embryonic, factors influencing differentiation pass into the regnerated tissue.3Tissue regenerated from the extreme tip retains embryonic features and consists of undifferentiated, apolar, callus cells.4In the subsequent zone, which later on gives rise to the apophysis, other factors are present, and the regenerating tissue gives rise to new setae.5The 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.6Although callus cultures give rise to protonema in darkness, a specific differentiation factor is present since callus is invariably reformed on illumination.7All 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.8The 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.