In root endodermis, layer of which substance provide ability to actively transport ions in one direction only.

Answer

### Step-by-Step Solution:

1. **Identify the Structure**: The question pertains to the root endodermis, which is the innermost layer of the cortex in roots.

2. **Understand the Composition**: The endodermis is characterized by its barrel-shaped cells that are tightly packed and have thickened walls.

3. **Recognize the Special Features**: Within the endodermis, there are specific structures known as Casparian strips or Casparian thickenings. These are lignified and suberized layers found on the radial and tangential walls of the endodermal cells.

4. **Function of Casparian Strips**: The primary function of the Casparian strips is to regulate the movement of water and minerals. They create a barrier that prevents the passive flow of substances, thus allowing for selective and active transport.

5. **Active Transport Mechanism**: The presence of suberin in the Casparian strips enables the endodermis to actively transport ions in one direction only, preventing backflow into the cortex.

6. **Conclusion**: The substance that provides the ability to actively transport ions in one direction in the root endodermis is **suberin**.

### Final Answer:
The layer of **suberin** in the root endodermis provides the ability to actively transport ions in one direction only.

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