Solution

The hypocotyl is the portion of seedling, where the transition of root structure into stem structure occurs. The seedling above the transition point has stem structure, which is characterized by the presence of collateral and endarch vascular bundles. And the region of hypocotyls below the transition point is structurally root, which bears radial and exarch vascular bundles. The transition point marks this conversion from exarch to endarch vascular bundles.