The arrangement of leaves on stems, the ‘phyllotaxis,’ is orderly: this is expressed by the predictability of the location of a leaf from the location of other leaves (Figs 11.1, 11.2). There are various phyllotactic patterns (Green and Baxter, 1987), and these depend on the species and often also on the stage of plant development and environmental conditions. Phyllotactic patterns are prominent, macroscopic and obvious in mature tissues, and they have remarkable mathematical properties. For these reasons they have received much attention, perhaps as much as any other biological pattern.

Most work on phyllotaxis has dealt with the mathematical or geometrical aspects of leaf arrangement (Church, 1904; Iterson, 1907; Richards, 1951; Mitchison, 1977; Erickson, 1983; Jean, 1984). These have been especially concerned with the mathematical properties of the different angles of divergence between leaves and of the lines connecting neighboring leaves (the parastichies), but this aspect of phyllotaxis is not directly related to the purpose of this book (Chapter 1). There have also been quantitative models of phyllotaxis that have supported detailed mechanisms concerning the relations between leaves – but these models have been based on unrealistic assumptions which bear no relation to actual plant development: assumptions that leaves of seed plants are not in direct contact at the time they are formed, that their bases are round and that these bases are passive, unchanging participants in apical events.