Introduction

Chapter 5 has already introduced how a flowering plant is organised — its roots, stems, leaves, flowers, fruits and seeds. This chapter turns to a related question: how and in what order do those structures actually arise? A seed becomes a seedling, then a plantlet, and eventually a mature plant, and every one of those stages is a familiar term, but the underlying sequence is worth examining closely. Trees, for instance, keep adding to their height and girth for as long as they live, yet the leaves, flowers and fruits growing on the very same tree stay within a limited size range and appear — and fall — periodically, sometimes over and over again in a plant's lifetime. Why should a plant pass through a vegetative phase before it ever flowers?

A related puzzle concerns structure and function. Every plant organ is built from a variety of tissues, and there is a real relationship between the structure of a cell, of a tissue, of an organ, and the job each performs — a relationship that can, under the right conditions, itself be altered. All of a plant's cells trace their ancestry back to a single zygote, so why and how do cells that share the same origin end up so different from one another in both structure and function?

The answer lies in two linked processes together called development: growth, and differentiation. Starting from the zygote, a mature plant's body is built up through a precise, highly ordered sequence of events that produces roots, branches, leaves, flowers, fruits and seeds in turn, before the plant eventually dies. The very first event in this sequence is seed germination, which begins only once the surrounding environment offers conditions favourable for growth. If those conditions are not met, the seed does not germinate at all — instead it enters a period of suspended growth, or rest, and only resumes its metabolic activities and growth once favourable conditions finally return.

This chapter examines the factors that govern and control these developmental processes — factors that are both intrinsic (arising from within the plant) and extrinsic (imposed by its environment).