(a) Auxin: 1. Auxins are well known to promote the elongation of the stem. It promotes the growth by elongation of cells behind the apical meristem. 2. Growth in the lateral bud is inhibited when the apical bud of a tall plant remains intact. However, the lateral bud grows rapidly on the removal of apical bud. 3. Suppression of growth in the lateral bud by apical bud due to auxin produced by apical bud in termed as apical dominance. The reason for this is due to auxin produced in growing tip and it stimulates growth but as it moves downwards, suppresses growth in the stems below. 4. Auxin is responsible for initiation and promotion of cell division in the cambium, which is responsible for secondary growth. This property of induction of cell division has been exploited for tissue culture techniques and for the formation of callus. 5. Auxin promotes the growth of root only at extremely low concentrations. At higher concentrations, it always inhibits the growth of the root. (b) Gibberellins: Gibberellin is one of the plant growth hormone which is required for the growth and development of the plants. The regulator is required for different processes. � The hormone causes elongation in the stem. � The hormone is responsible for the bolting effect in the rosette plants. � The regulator is externally used for the removal of dormancy in the case of seeds and buds. � Ther hormone promotes and stimulates seed germination. � The hormone can be used as an alternative to vernalization. � They promote flowering in the long day plants. (c) Cytokinins.: Cytokinins induce cell division and promote the growth of developing shoot buds, root apices, young fruits, etc. It promotes the development of lateral shoot and thereby helps in overcoming apical dominance. Cytokinins delay leaf senescence by promoting nutrient mobilisation.