A series of experiments involving ovules cultured at different times around anthesis was conducted to investigate the timing of differentiation of ovule epidermal cells into prefiber cells. IAA, GA and a high CO₂ atmosphere (5% was used) were essential for preanthesis ovules from greenhouse plants to produce fibers when cultured on a modified Beasley-Ting medium. The permutation of experiments showed that epidermal cells are capable of becoming fibers between 3 and 2 days preanthesis. Fibers could be initiated any time after the potentiation event through the in vivo scheduled time of anthesis. The fiber potential of ovules cultured preanthesis was lost at the equivalent time of anthesis in situ.