Phyllotaxis and Organogenesis in Ferns

Abstract

FROM experimental studies of phyllotaxis in flowering plants. M. and R. Snow¹ concluded that the next leaf Primunordium to be formed will arise in the “next available space’ on the apical meristem, that is, tha first space or gap between existing primorditf that attains a certain minimum width and which is situated a certain distance below the extreme tip of the shoot. They showed that the position of the new primordium within a gap can be modified by experimental treatment and that it is determined by those primordia which abut on the gap and not by all the primordia of the top cycle. In comparable experiments with. Dryopteris aristata Druce, I obtained evidence which substantially supports these findings². In ferns, an evident feature is that the newly formed primordium occupies only a small part of the interfoliar meristematic area. There is evidence that tensile stresses are induced in the fern apical meristem by the existing leaf primordia, new primordia typically arising in regions of minimal stress. Accordingly, it has been suggested that these stresses may determine, or define, the space in which the next primordium can be formed ; but that other factors are probably responsible for leaf formation².