The formation process of rhizome to tuber in Apios americana Medikus (apios, groundnut) was investigated. The rhizome is a stem, which is shown by its morphological and anatomical structures. It consists of several nodes and internodes, stipule-like structures, scale-like leaves with axillary buds, and a hook-shaped tip. Several roots were developed on the rhizome just at the basal side of each node. Tubers of apios formed with the swelling of each internode of the rhizome, and they developed at basal internodes through internodes near the rhizome tip. In general, the first tuber developed at the 1st through the 4th internodes of rhizome. Three developmental processes could be recognized during tuberization in each internode: (I) tuber initiation, (II) extension of swelling areas, and (III) dominant enlargement of tuber. Tuberization took place at 8 weeks after planting. The swelling of internodes initiated at the basal side of the axillary bud of the node, extended basipetally and enlarged laterally. The swelling areas of the internodes coincided with the location of root development which occupied 20 to 50% of the length of the internode. Thus, a whole internode did not always swell. In a field, however, a whole internode or several successive short internodes were sometimes tuberized.
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