BRIDGES, BRANCHES, PLANTS AND PROTISTS

At a developmental biology meeting, in front of a poster advertising somewhat generally the activities of my group on kinetic theories of pattern formation, I was asked by a plant geneticist: how do plant meristems manage to break circular symmetry? I was rather delighted to hear such physicists' terminology as ‘symmetry-breaking’ used in such a connection by a biologist. To my disappointment, I was unable to expand this incident into a collaboration; but for a moment this interaction was certainly right in the middle of the kind of bridge I keep trying to build. What such symmetry-breaking leads to is branching, the most common and well-known feature of plant growth, the thing that makes a bush bushy or a flower floral. And, despite the vast amount of information that is piling up on genetics, with all its powerful applications, we don't know what makes branching start, i.e. what makes apparently identical portions of a simple shape behave differently to make the shape more complex. Turing (1952) devised his theory of morphogenesis having in mind examples in the animal kingdom, especially the loss of spherical symmetry in a blastula when gastrulation commences. For plants, I return to the discussion of symmetry in my account of dichotomous branching in Chapter 4, with the idealized starting point of a hemispherical meristem topping a cylinder.