Question

why we immediately put the peel of onion bulb in a water containing pertisdish

Answer 1

we immediately put the onion bulb in water containing Petridish just because water washes away all dirt and extra impurities from it to give a clear look under microscope...
hope this answer helps u....

Answer 2

any living cell undergoes osmosis; that is the movement of water from an area of higher concentration to an area of lower concentration. when you expose a peel of onion bulb to air, the water tends to move out because the concentration of water is more inside the cell as compared to its surroundings. thus, plasmolysis or the shrinking of the cell membrane could occur. to prevent this, we put the eel immediately in a Petri dish containing water.