Preface

Abstract

Contributors to this Discussion Meeting were given only the most general guidance to what is meant by ‘modular’. The letter of invitation said “By ‘modular’ we imply growth of the genetic individual by the repeated iteration of (multicellular) parts - modules. This means that we can consider together at the meeting the growth and form of corals, higher plants, fungi,..., ascidians, ..., bryozoans, etc.” . Modular growth contrasts with ‘unitary’ growth, in which the zygote develops to a determinate structure that is repeated only when a new life cycle is started from a single-celled stage, usually a zygote. Of course most unitary organisms bear repeated parts, such as legs, wings and body segments, but they are formed in early embryogenesis, not by continued or prolonged somatic iteration, and their numbers are usually very strictly determinate. We might count the numbers of flies in a population of Drosophila by counting the numbers of legs and dividing by six, or of birds by counting the wings and dividing by two. There is no such determinate divisor for the genetic individual (genet) of a modular organism - no determinate number of leaves or buds on a tree, polyps on a coral, hyphae in a fungal colony or branches on a root system. Modules themselves, however, may be quite strictly determinate in form, e.g. the number of petals on a flower, the number of leaflets on a leaf.