Solution

Skoog and Miller showed in tobacco callus cultures that organ formation depends on the relative amounts of auxin and cytokinin in the medium.
A high cytokinin : auxin ratio promotes the differentiation of shoots (buds).
A high auxin : cytokinin ratio promotes root formation.
Roughly equal amounts keep the callus growing without differentiation.
So differentiation of shoots is controlled by a high cytokinin : auxin ratio.

Concept Cytokinin to auxin ratio in organogenesis

Plant Growth and Development in past papers

85 questions from this chapter have appeared across 24 exam years.