During the investigation of callus proliferation in tobacco stems, Miller et al. identified and crystallised a cytokinesis-promoting active substance termed kinetin. Based on its chemical composition, this plant growth regulator is classified as:

Step-by-step solution

Miller et al. identified and crystallised kinetin, which is a cytokinesis-promoting active substance (a cytokinin). According to the chemical composition of plant growth regulators, cytokinins like kinetin ( N⁶ -furfurylamino purine) are modified purines, specifically adenine derivatives. Indole compounds refer to auxins (e.g., IAA), terpenes refer to gibberellins (e.g., GA₃ ), and derivatives of carotenoids refer to abscisic acid (ABA). Answer: An adenine derivative