When a freshly prepared Spirogyra filament is kept in 10% potassium nitrate solution, then the protoplasm is observed to shrink in size because of the phenomenon of exosmosis.

a) This phenomenon is called exosmosis: process by which when a substance is placed in hypertonic solution, the water moves out of the cell of the substance to the outside leading to the shrinkage of the protoplasm.

b) If the filament is replaced in distilled water, then the protoplasm again becomes turgid as water flows in as the shrunken protoplasm swells up or gets deplasmolysed.