Characteristics of hook formation by bean seedlings.

Explants were isolated from 6-day-old etiolated bean seedlings (Phaseolus vulgaris L. cv. Black Valentine) containing the cotyledons with 4 mm of hypocotyl just below the node and/or the epicotyl. During incubation on distilled water, uneven growth of the hypocotyl or epicotyl occurred resulting in the formation of a hook. The more rapid growth of the side which became convex was not dependent upon the presence of the slower growing concave side. It was concluded that the main axis has an intrinsic capacity for asymmetric growth. The growth leading to hook formation was inhibited by alpha-naphthaleneacetic acid at concentrations above 0.2 milligram per liter.