Growth Substances in Higher Plants

Growth substances may be defined as-organic compounds which at low concentrations promote, inhibit, or qualitatively modify growth. Their effect does not depend on their caloric value or their content of essential elements. By “low concentrations” is generally meant concentrations lower than 10−4 molar when the compound is applied externally in aqueous solution to suitable test objects. The so-called bios substances (e. g. thiamin, biotin, etc.) are treated individually in other chapters of this handbook. The main subject of the present chapter, therefore, II be substances which affect the extension of the cell wall, accompanied by water uptake in the cell. Such substances are classified according to the qualitative character of their biological effect. There is as yet no general agreement on the boundaries and definitions of the individual groups of growth substances. Some natural groups are listed below, and the definitions which will be used in the present chapter are given.