After boiling the cell would have lost its water (dehydrated).It appears like a deflated balloon. Now it needs water to regain its original form or swell. When it is placed in hypotonic med. the water from the medium enters into the cell .Hence it regains its size. The process occurred here is called “endosmosis”. This is due to a concentration gradient of water between the medium and the cell. Excess water enters into the cell which has less water or say no water at all.
