Question

What are passage cells?

Answer 1

Hint: The cells are located in the endodermal region of the roots in the close proximity of the xylem elements. They perform the same function as the term in their name ‘passage’ suggests.

Complete answer:
Passage cells are endodermal cells of older roots that have retained thin walls and Casparian strips instead of becoming suberized and waterproof just like the other cells around them, to continue to permit some symplastic flow to the inside. Experimental evidence suggests that The passage cells function to permit the transfer of solutes like calcium and magnesium into the stele, this has been suggested by experimental evidence. To eventually reach the transpiration system. For the foremost part, however, old roots seal themselves off at the endodermis, and only function a passageway for water and minerals haunted by younger roots "downstream".Thus these cells are referred to as passage cells thanks to its function to move water and minerals salts from the cortex to the xylem elements.
The endodermis prevents water, and any solutes dissolved within the water, from passing through this layer via the apoplast pathway. Water can only undergo the endodermis by crossing the membrane of endodermal cells twice (once to enter and a second time to exit). Water getting into or out of the xylem, which is a component of the apoplast, can thereby be regulated since it must enter the symplast within the endodermis. This allows the plant to regulate to a point the movement of water and to selectively uptake or prevent the passage of ions or other molecules.

Note: -The endodermis does not allow gas bubbles to enter the xylem and helps prevent embolisms from occurring in the water column. Endodermal cells may contain starch granules in the form of amyloplasts. These may function food storage and are shown to be involved in gravitropism in some plants.
-The Casparian strip is a band of cell wall material deposited in the radial and transverse walls of the endodermis. It is chemically different from the rest of the cell wall.