Every such corporation shall have power to provide by its bylaws for succession to its original membership and for new membership, and also for the election from its members of a board of trustees, or a board of directors, and to fix the number and term of office of such trustees or directors; PROVIDED, That there shall always be upon such board of trustees or board of directors at least one representative from each of the several bodies represented in the membership of the association, and the term of office of a trustee shall not exceed three years.