Root Collar

the transition zone between the root and the stem; it separates the hypocotyl of a plant from the primary root. At times this area is greatly swollen and clearly visible, particularly in seedlings; in mature plants the root collar is difficult to recognize. In agricultural practice the root collar is frequently mistakenly thought to be the thickened hypocotyl located above it or even the epicotyl. As a rule, the root collar is in the soil, and many secondary roots form in that zone.