The board of directors shall be elected at the annual membership meeting, and shall take office immediately upon its adjournment. The board of directors shall consist of 21 citizens, elected by the compact membership as follows: three classes of seven members each, initially elected for terms of one, two, and three years respectively, and thereafter elected for terms of three years each. Vacancies shall be filled by written, mail ballot of the membership. At least five of the members elected shall be black. No member shall be eligible to serve more than two three-year terms consecutively.