Section 32. Stockholders entitled to vote shall, except as provided in sections two and four of chapter one hundred and fifty-seven, have one vote for each share of stock owned by them; provided, that in corporations having two or more classes of stock, the voting powers of the different classes may be fixed in the manner provided by section fourteen. Capital stock shall not be voted upon if any instalment of the subscription therefor which has been duly demanded under section eighteen is overdue and unpaid. Stockholders may vote either in person or by proxy. No proxy which is dated more than six months before the meeting named therein shall be accepted, and no such proxy shall be valid after the final adjournment of such meeting.