In this chapter, I describe detailed experimental data relevant to mechanisms of pattern formation for two biological systems that I have studied. For the Acetabularia whorls, I believe the evidence points strongly to a reaction–diffusion mechanism; for the Larix cotyledons, it indicates strongly that the mechanism is waveforming and hence that the embryo apex is behaving as a coherent whole during pattern formation. However, the evidence does not distinguish between reaction–diffusion and mechanical buckling. I reserve some account of the latter until the following chapter (Section 4.2); the mechanism is, however, equally applicable to dichotomous branching and whorls of a larger number of parts.

ACETABULARIA ACETABULUM VEGETATIVE WHORLS

The morphological event

Trying to cross disciplinary boundaries, one is beset with many perils. Some of these are rather trivial things to do with word usage. Having read often of ‘whorls of stamens’ and suchlike, I had assumed without the benefit of a dictionary that ‘whorl’ meant a set of structures arranged in a ring. I have recently been warned that to some biologists, the whorl is only the ring itself. I have been calling that an annulus, and the set of structures disposed around it the whorl. In this book, I shall adhere to that usage, deferring to the other one only in the title of this chapter.