(a) Plasmolysis is the name of the phenomenon, occurring is Spirogyra filament when placed in 10\% potassium nitrate solution (hypertonic solution). It occurs as water from the cell is drown out to extracellular fluid causing the protoplast to shrink away from cell wall.

(b) When the Spirogyra again reabsorb water, protoplast will again and will come back in its original shape. This phenomenon is known as deplasmolysis.

Pure water has tremendous free energy at normal room temperature. That is called water potential. When the water temperature drops down, the free energy is reduced so, sugar does not get dissolved easily in, i.e., cold water.

Salt solution being hypertonic, causes exo-osmosis in plants. 1 cup salt in 2 cup of water, fairly well dissolved when sprayed on weed plants, start killing them. This salt concentration is then increased in subsequent sprays, if the weeds are still thriving in the field.

Xylem sap consists of mainly water and some solutes or mineral nutrients.

Phloem sap mainly consists of the sucrose, a kind of transport sugar which is a diasaccharide, hormones and amino acids are also found in phloem sap of the plants.

The tube $B$ will show higher level than tube $A$ because of the capillary action.