TUBERIZATION IN APIOS (APIOS-AMERICANA MEDIKUS) .1. DEVELOPMENTAL MORPHOLOGY OF TUBER

The formation process of rhizome to tuber in Apios americana Medikus ( apios, groundnut) was investigated. The rhizome is a stem, which is sh own by its morphological and anatomical structures. It consists of sev eral nodes and internodes, stipule-like structures, scale-like leaves with axillary buds, and a hook-shaped tip. Several roots were develope d on the rhizome just at the basal side of each node. Tubers bf apios formed with the swelling of each internode of the rhizome, and they de veloped at basal internodes through internodes near the rhizome tip. I n general, the first tuber developed at the Ist through the 4th intern odes of rhizome. Three developmental processes could be recognized dur ing tuberization in each internode: (I) tuber initiation, (II) extensi on of swelling areas, and (III) dominant enlargement of tuber. Tuberiz ation 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 interno des 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 succe ssive short internodes were sometimes tuberized.