Abstract: HIGHER plants show such flexibility in their growth responses to environmental influences that they must have a very complex internal system controlling the processes involved. How otherwise could weeds show such powers of regeneration after being cut down, or plants in dark corners grow towards the light, or leaves and cuttings grow into new plants ? Plant physiologists are now beginning to build up a picture of how this flexibility could be provided by a system of interacting growth promoters and inhibitors. These hormonessubstances produced in one part of a plant and transported to another where they exert their influenceare the well known auxins, gibberellins and cytokinins which promote growth, and the newly discovered abscisic acid, otherwise known as dormin becamie of its ability to slow down plant growth. Of these hormones, the cytokinins have proved the most difficult to isolate from living plants. After 6-furfurylaminopurine, a derivative of adenine, was found to induce plant cells to divide, and named kinitin in 1955, several other synthetic cytokinins of similar structure were produced. The first naturally occurring cytokinin, called zeatin, was isolated from maize kernels, and Carlos Miller in Wisconsin has recently reported finding three other substances with the properties of cytokinins in these seeds1 . Several naturally occurring auxins have been isolated apart from indoleacctic acid (IAA) which is widely distributed. All the naturally occurring auxins are chemically very similar to IAA and include indoleacetonitrile and indoleacetaldehyde. Since the 1920s, when a growth substance was first isolated in Japan from the fungus Gibberella fujikuroi which was causing excessive growth in its host rice plants, gibberellins have been found in many higher plants. All twenty-two of the known gibberellins have chemical structures similar to that of gibberellic acid which is often used in experimental work. The latest known growth regulator, abscisic acid, was first found in extracts from cotton bolls and identified as a promoter of the abscission of cotton leaves. Later abscisic acid was found to encourage dormancy (hence the name dormin) and is now known to be ubiquitous in higher plants.