A systematic ultrastructural study of the apical meristem of Spinacia oleracea has been performed at the time of the transfer from photoperiodic conditions unfavourable to flower induction (short days) to favourable photoperiodic conditions (long or continuous days). A precise sequence of revision is evident, particularly in the chloroplasts and mitochondria. The changes, which are characteristic of different apical areas, occure before the reported modifications in the nucleic acids (nuclear, ribosomal), and are related to the evocation process.