AbstractThe protonema ofOrthotrichum obtusifolium,regenerated from leafy shoots, comprises a dense mass of largely unbranched exclusively upright filaments. All the cells have transverse cross-walls and contain numerous chloroplasts which become larger and discoidal in older filaments. Differentiation into chloro- and caulonema is absent. When upright filaments are transplanted onto an agar surface, they act as the main axes for a further set of upright branches. Buds and side branches are produced symmetrically at right angles to the mother cells. Protonemal morphogenesis hardly changes on nutrientfree medium and spherical brood cells are not produced after prolonged culture or in the presence of abscisic acid. Rhizoids with oblique septa and pigmented walls arise from the base of gametophores. These revert to protonemata when in contact with nutrient agar.The upright filaments produce terminal and intercalary filamentous gemmae with well defined abscission cells. The latter swell and lose their contents prior to liberation. Cells immediately below the abscission cells produce side branches in acropetal succession. Secondary gemmae arise by percurrent proliferation and gemmae frequently germinatein situ.