Any of a group of plant hormones chemically related to the purine adenine. Cytokinins are involved in numerous aspects of plant metabolism. They stimulate cell division in the presence of auxin and have also been found to delay senescence, overcome apical dominance, and promote cell expansion. They are produced by growing roots in spring and translocated to the stem, where they antagonize the effects of auxin and strigolactone to stimulate growth of buds. The endosperm of seeds contains high concentrations of cytokinins, which are thought to be involved in development of the embryo. Zeatin is a naturally occurring cytokinin. Carefully formulated mixtures of cytokinins and auxins are used in micropropagation to generate cloned plantlets from undifferentiated callus tissue; the auxin:cytokinin ratio controls whether the callus develops shoots or roots, and also the extent of branching in a plant.