Gynophore elongation, pod formation and pod orientation in the peanut plant (Arachis hypogaea L.) were studied in relation to the effects of light and dark conditions, mechanical stimulus, and growth substances. It was found that the proembryos control gynophore elongation, probably by secretion of growth regulators which stimulate cell division in the intercalary meristem located proximal to the ovules. The stimulus of pod production causes the development of the proembryo into a mature embryo simultaneously with the growth of pod tissues and the cessation of gynophore elongation. Darkness was found to be an essential factor for the induction of pod formation. Pod formation did not occur in any of the treatments performed in the light, including the application of different growth substances on the ovary. A mechanical stimulus is needed, in addition to darkness, for the normal thickening and diageotropic orientation of the pod, caused by a higher growth rate of the basal proximal side of the pod. The two ovules are always located on the upper wall of the diageotropically oriented pod (ventral suture). A possible mechanism which causes such an orientation is discussed.
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Zamski et al. (1976) studied this question.