In all elections of directors, each shareholder shall have the right to vote the number of shares owned by him for as many persons as there are directors to be elected, or, if so provided by the articles of association of the national bank, to cumulate such shares and give one candidate as many votes as the number of directors multiplied by the number of his shares shall equal or to distribute them on the same principle among as many candidates as he shall think fit; and in deciding all other questions at meetings of shareholders, each shareholder shall be entitled to one vote on each share of stock held by him; except that (1) this shall not be construed as limiting the voting rights of holders of preferred stock under the terms and provisions of articles of association, or amendments thereto, adopted pursuant to the provisions of section 51b of this title; (2) in the election of directors, shares of its own stock held by a national bank as sole trustee, whether registered in its own name as such trustee or in the name of its nominee, shall not be voted by the registered owner unless under the terms of the trust the manner in which such shares shall be voted may be determined by a donor or beneficiary of the trust and unless such donor or beneficiary actually directs how such shares shall be voted; and (3) shares of its own stock held by a national bank and one or more persons as trustees may be voted by such other person or persons, as trustees, in the same manner as if he or they were the sole trustee. Shareholders may vote by proxies duly authorized in writing; but no officer, clerk, teller, or bookkeeper of such bank shall act as proxy; and no shareholder whose liability is past due and unpaid shall be allowed to vote. Whenever shares of stock cannot be voted by reason of being held by the bank as sole trustee such shares shall be excluded in determining whether matters voted upon by the shareholders were adopted by the requisite percentage of shares.