Observations made on the effects of auxin, of cytokinin and of various light treatments on cultures of wild-type Physcomitrella patens, grown either on solid medium or continuously-replaced liquid medium, have allowed us to suggest probable roles for these factors during the early stages of normal gametophyte development. Further data obtained by studying mutants affected in their responses to one or both types of hormones or to light have supported our model of gametophyte development and enabled us to elaborate it.