Introduction

The life cycle of higher plants is regular, though complex. Each stage, together with the shift to the next stage, is controlled by endogenous plant growth regulators. The situation is further complicated by the need for the life cycle to accomodate to environmental conditions such as light intensity, daylength, humidity, and nutritional conditions. The concept of plant hormones differs substantially from that of hormones in animals and insects, because the differentiation of organ tissues in plants is less extensive than in animals. Plant hormones can be broadly defined as follows: they must be chemically characterized and shown to be biosynthesized in some plant organ, they must be broadly distributed within the plant kingdom, they must show specific biological activity in very low concentration and must be shown to play a fundamental role in regulating physiological phenomena in vivo, and they are usually translocated within the plant from a biosynthesis site to an action site.