What is the meaningof contactile vacuole???What is the meaningof reservoirs

Contractile vacuole means vacuole which can change its shape as required. In amoeba they are found for excretion .as amoeba have to excrete something these waste material come in contact with these vacuoles and by slowly changing in shape amoeba exrete it. RESERVOIR means which can reserve something as in case of plant vacuoles where ions and many useful things are reserved for future use of plants.