Casparian strips are present in :

Question

Accepted Answer

The correct option is 3: Endodermis

The correct option is 3: Endodermis. The **Endodermis** is a highly specialized, single layer of cells that forms the innermost boundary of the cortex in plant roots. It encircles the central vascular cylinder, or stele, and acts as a critical biological checkpoint regulating the flow of substances. The defining characteristic of the endodermis is the presence of the **Casparian strip**. This strip is a band-like thickening that runs completely around the radial (vertical) and transverse (horizontal) cell walls of each endodermal cell. It is made of hydrophobic, waxy substances, primarily suberin and sometimes lignin, which are impermeable to water. The Casparian strip's waterproof nature compels water and dissolved minerals to abandon the *apoplast* pathway (movement through cell walls and intercellular spaces) and enter the *symplast* pathway, forcing them to cross the living plasma membrane of the endodermal cells. This ensures that the plant can actively select and regulate which substances move from the soil into the water-conducting xylem tissue.

The incorrect option is 1: Pericycle.
The **Pericycle** is the layer of cells situated immediately *inside* the endodermis, which means it constitutes the outermost layer of the central vascular cylinder (stele). Its primary functions are developmental, as it is the tissue from which lateral roots originate, and it contributes to the formation of the cork cambium during secondary growth. Unlike the endodermis, the pericycle is fully permeable to water and dissolved solutes; it does not possess the Casparian strip, which is a feature of the protective boundary layer outside of the stele. Therefore, placing the Casparian strip in the pericycle is incorrect both structurally and functionally, as the strip's job is done before that layer.

The incorrect option is 2: Xylem.
The **Xylem** is the main vascular tissue responsible for the long-distance, upward transport of water and dissolved minerals from the roots to the shoots and leaves. The conducting cells of the xylem, such as tracheids and vessels, are non-living, hollow tubes. The entire function of the Casparian strip is to regulate what enters the xylem; thus, the strip is a barrier placed *outside* the transport system, not a component of the transport system itself. If the Casparian strip—a water-impermeable structure—were present in the xylem, it would block the flow of water and minerals entirely, which would be contrary to the xylem's vital function as a conduit.

The incorrect option is 4: Cortex.
The **Cortex** is the region of the root situated between the epidermis and the central vascular cylinder. It is a broad area composed primarily of parenchyma cells whose functions include storage of food and water. While the endodermis is, developmentally, the innermost layer of the cortex, the term "Cortex" is anatomically general and includes all the layers outside the endodermis. The Casparian strip is a specialized, unique feature that is strictly localized to the cell walls of the endodermis, not throughout the entire, multi-layered cortex tissue. Therefore, attributing the Casparian strip to the entire cortex is imprecise and fails to identify the correct, single, highly specialized layer.