§ 21. Officers. The officers of a cooperative shall consist of a president, vice-president, secretary and treasurer, who shall be elected annually by and from the board of directors. When a person holding any such office ceases to be a director he shall cease to hold such office. The offices of secretary and of treasurer may be held by the same person. The board of directors may also elect or appoint such other officers, agents, or employees as it deems necessary or advisable and shall prescribe their powers and duties. Any officer may be removed from office and his successor elected in the manner prescribed in the by-laws.