The directors may appoint one of their number to be the chairman of the board of directors and may at any time remove him from that office. Unless he is unwilling to do so, the director so appointed shall preside at every meeting of directors at which he is present. If there is no director holding that office, or if the. director holding it is unwilling to preside or is not present within five minutes after the time appointed for the meeting, the directors present may appoint one of their number to be chairman of the meeting.