Apoplastic water flow blocked by Casparian strip in roots

Steps:

• Water enters root hairs and moves through cell walls of cortex via apoplast pathway.
• Flow reaches endodermis, the innermost cortex layer.
• Casparian strip in endodermis, a suberin band, blocks apoplastic continuation.
• Water rerouted through symplast via cell membranes and plasmodesmata.

Why A is correct:

• Casparian strip is a waterproof band of suberin in endodermal walls that forces apoplastic water to cross plasma membranes, per root transport physiology.

Why the others are wrong:

• B: Casparian strip is impermeable to apoplastic water, preventing easy passage.
• C: Plasmodesmata enable symplastic flow after Casparian block, not the arrowed apoplastic path.
• D: Endodermis resists easy water passage due to suberin barrier, requiring membrane crossing.

Final answer: A