It has been suggested that peg formation on the transition (TR) zone between hypocotyl and root in Cucurbitaceae seedlings is regulated by gravity. We have examined whether the TR zone of cucumber seedlings possess a gravity-sensing apparatus which might be involved in peg formation. The results showed that there are sedimentable amyloplasts in the sheath cells and adjacent cells to them in the TR zone, which appear well before negative gravitropic bending of the hypocotyl occurs. Because the cortical cells do not have significant number of sedimenting amyloplasts, the gravity-sensing mechanism for peg formation may be similar to that for gravitropism. Some chemical stimulus may migrate from the gravisensing cells to the cortical cells which are destined to become a peg by the alteration of growth polarity. An increased auxin level in the cortical cells appears to be a direct cause of peg formation. Thus, peg formation of Cucurbitaceae seedlings may be a pleiotropic response to the graviperception and physiological changes that function in shoot gravitropism.