Introduction

In addition to the descriptive aspects of phyllotaxis (Part I), there are more fundamental matters concerned with the causes inducing phyllotactic patterns. How do plants achieve such pattern formation, especially when sometimes apparently quite extraordinary precision is attained, as we have seen? Where does the control come from that establishes the beautiful natural patterns described here? There are several hypotheses and mathematical models designed to explain the dynamical and causal aspects of pattern formation in phyllotaxis. The main hypotheses and/or models can be roughly categorized as:

1. Physical. Snow and Snow's first available space hypothesis (1962) that predicts the formation of a new primordium, whenever by growth a certain minimal amount of space becomes available at the apex. Esau's (1965b) and Larson's (1977) procambial strands hypothesis insisting on the acropetal influence of the vascular traces in the determination of leaves and in pattern formation. Plantefol's (1948,1950) hypothesis (see Section 2.6) which calls upon Snow and Snow's in order to explain some observations. Schwendener's (1878) hypothesis and Adler's (1977a) model that base pattern formation on mutual physical contact pressures between primordia. Green's reinforcement patterns of cells (1985) and bulging phenomenology of the tunica (1992), for which patterns are the results of surface stresses between cells.