When a freshly collected $Spirogyra$ filament is kept in a $10\%$ potassium nitrate solution,it is observed that the protoplasm shrinks in size.
$(a)$ What is this phenomenon called?
$(b)$ What will happen if the filament is replaced in distilled water?

(N/A) The phenomenon is called plasmolysis. When the $Spirogyra$ filament is placed in a $10\%$ potassium nitrate solution (a hypertonic solution),water moves out of the cell due to osmosis,causing the protoplasm to shrink away from the cell wall.
$(b)$ If the filament is replaced in distilled water (a hypotonic solution),the water will move back into the cell through osmosis. This causes the cell to regain its turgidity and return to its original state,a process known as deplasmolysis.