A cooperative may issue to its members certificates of membership. Only members shall be entitled to vote at the meetings of the members of the cooperative. A member entitled to vote may vote in person or, unless the articles of incorporation or the bylaws otherwise provide, may vote by proxy executed in writing by the member, appointing some other member to cast his vote, or may vote by his duly authorized attorney-in-fact, who shall be a member. If a member is not voting in person, by proxy, or by attorney-in-fact, the spouse of such member shall be entitled to vote for the member, in person or by proxy, if permitted by the articles of incorporation or bylaws. No proxy shall be valid after eleven months from its date unless otherwise provided in the proxy. When directors or officers are to be elected by members, the bylaws may provide that such elections may be conducted by mail. The liability of each member shall be limited to the unpaid portion of his membership fee or subscription to capital stock, and any unpaid bills for telephone service from the cooperative. The equity of members of a nonstock cooperative shall be in proportion to the revenue paid the cooperative by each member. A cooperative shall be operated on a nonprofit basis for the mutual benefit of its members. The bylaws of the cooperative or its contract with its members shall contain such provisions relative to the disposition of revenues and receipts as may be necessary and appropriate to establish and maintain its nonprofit and cooperative character.