THE CONTROL OF SHOOT GROWTH AND OF FLOWERING BY PLANT GROWTH-REGULATING SUBSTANCES
Formerly, the growth and flowering of pot plants were regulated mainly by control of their physical environmental conditions and their nutrition. The extreme example is the japanese dwarf-trees, that are formed into exactly determined stunted shapes and yet bear flowers and fruits. In modern glasshouses, temperature, humidity, and illumination conditions can be controlled, and the nutritional factors can be provided by the pot soil and by sprinkling.
However, the way by which these factors influence vegetative growth and generative development is largely indirect. While water supply, mineral nutrition and photosynthesis are required for the development of shoots and flowers, the internal control of this development is exerted by hormonal compounds. The internal levels of these regulating substances are partly genetically determined, partly influenced by the environmental conditions. From their interplay results the direction in which the development of a plant proceeds. It is the balance between the various plant hormones that determines whether a seed remains dormant or germinates, whether a plant remains vegetative or becomes generative, whether a plant or an organ is juvenile, mature, or senescent.
Both natural and artificial plant growth-regulating substances have become available. They enable the grower to interfere directly with the internal hormonal balance of his crop plants, either by inhibiting the bio-synthesis of hormonal substances or by influencing their activity or breakdown. The application of these direct means may lead to dangerous derailments, the prevention of which requires some understanding of their mode of action. It is the aim of the present paper, primary, to deal with the regulation of shoot growth and flowering by the internal hormonal balance of the plant, and secondary, to touch upon the practical implications of this hormonal regulation.