vacancies.] The corporate powers, business and property of all railroad corporations must be exercised, conducted, controlled and managed by a board of not less than five nor more than eleven directors, to be elected from among the stockholders, who are citizens of the United States. Unless a quorum be present and acting, no business performed or act done, shall be valid or binding as against the corporation. Vacancies in the board of directors shall be filled by appointment by the board, unless otherwise provided by the bylaws of the corporation. Within the limits above specified, the number of directors may be increased or diminished by a vote of stockholders representing two-thirds of the subscribed capital stock, at any annual meeting of the stockholders.