The business and affairs of a dental service plan corporation shall be managed by a board of trustees, which shall have the power to adopt and to amend, from time to time, by-laws governing the conduct of the corporation’s business.The number of trustees shall be not less than 9 nor more than 21. Subject to such limitation, the number of trustees shall be fixed by the by-laws, except as to the number of the first board of trustees which number shall be fixed by the charter, and the number of trustees may be increased or decreased from time to time by amendment to the by-laws. In the absence of a by-law fixing the number of trustees, the number shall be the same as that stated in the charter. The trustees constituting the first board of trustees shall be named in the charter and shall hold office for such period or periods as may be specified in the charter or the by-laws. Trustees may be divided into classes and the terms of office of the several classes need not be uniform, but no such term shall exceed 3 years. Each trustee shall hold office for the term for which he is elected and until his successor has been elected and qualified. Any vacancy occurring in the board of trustees and any trusteeship to be filled by reason of an increase in the number of trustees may be filled by the board of trustees. A trustee appointed to fill a vacancy shall be appointed for the unexpired term of his predecessor in office. Each trustee shall be of legal age and a citizen of the United States. At least one-third of the trustees shall be participating dentists of the corporation licensed in Illinois to practice dentistry who are also residents of Illinois, and they shall be designated as “dental trustees”. The remaining trustees shall be designated as “public trustees”. As the terms of dental trustees expire, they shall be replaced, from time to time, by participating dentists of the corporation licensed in Illinois to practice dentistry who are also residents of Illinois, and elected by the corporation’s participating dentists, in the manner prescribed by the by-laws. As the terms of public trustees expire, they shall be replaced, from time to time, by election by the corporation’s subscribers, in the manner prescribed by the by-laws. Voting by proxy shall be permitted.