Which plant tissue layer acts as a barrier around the vascular cylinder in roots and regulates entry into the xylem?

Which plant tissue layer acts as a barrier around the vascular cylinder in roots and regulates entry into the xylem?

• A. Cortex
• B. Endodermis
• C. Pith
• D. Phloem

Answer: (B)

The layer that acts as a barrier around the vascular cylinder and regulates entry into the xylem is the endodermis. This single-cell-thick layer encircles the stele (vascular cylinder) and contains the Casparian strip, a band of suberin in the cell walls that blocks passive movement through the cell walls (the apoplast). Because of this, water and minerals cannot simply seep into the xylem; they must first cross the plasma membranes of endodermal cells, effectively regulating what gets loaded into the xylem and ensuring selective uptake. The cortex surrounds the endodermis and mainly serves storage and transport roles, not as a selective barrier. The pith sits in the center of the root and isn’t involved in controlling entry into the xylem. Phloem is part of the vascular tissue that transports sugars and is not the barrier to xylem entry.

The layer that acts as a barrier around the vascular cylinder and regulates entry into the xylem is the endodermis. This single-cell-thick layer encircles the stele (vascular cylinder) and contains the Casparian strip, a band of suberin in the cell walls that blocks passive movement through the cell walls (the apoplast). Because of this, water and minerals cannot simply seep into the xylem; they must first cross the plasma membranes of endodermal cells, effectively regulating what gets loaded into the xylem and ensuring selective uptake. The cortex surrounds the endodermis and mainly serves storage and transport roles, not as a selective barrier. The pith sits in the center of the root and isn’t involved in controlling entry into the xylem. Phloem is part of the vascular tissue that transports sugars and is not the barrier to xylem entry.