Apical size, phyllotaxis index and the rate of leaf inception were compared in spiral and bijugate specimens of Dryopteris from the same collection and were found to be similar. Apices were laid bare and maintained under observation in the laboratory for periods of up to 9 months; during this time the bijugate specimens became spiral, as previously reported. Exploratory experiments on the bijugate genus Dipsacus indicate that treatment with gibberellic acid leads to increased internodal elongation and a slight increase in the mean angle of divergence. Removal of young developing leaves and leaf primordia leads to an increase in the rates of leaf inception and development. These observations are discussed in relation to the earlier observations and experiments of Snow. The conclusion is reached that while apices with different systems of phyllotaxis may be of similar size and have similar rates of leaf formation and radial growth, progressive changes in various rates of growth may nevertheless lead to changes in phyllotaxis by modifying the relationships between successive leaf primordia. Vertical as well as radial and tangential growth should be taken into account.