In moss protonema of Funaria hygrometrica and other mosses the transition of chloronema to caulonema is regulated by external factors such as light intensity. In low light intensity this process is delayed or completely repressed. Under such conditions externally applied auxins (IAA, NAA, 2.4D) and auxin precursors (tryptophan, indolpyruvic acid, indolacetaldehyde) can induce the formation of caulonema. Indolacetonitrile and tryptamine are completely ineffective. Because the caulonema cells are the target cells for bud formation, buds can be formed only if auxin is applied before cytokinin. If kinetin is applied first and then removed before a subsequent auxin treatment, no bud formation results. This is interpreted as a sequential interaction, where auxin creates target cells for cytokinin. ABA inhibits growth and caulonema formation.