Section 72. Cities and towns may make ordinances and by-laws for carrying out the purposes of section seventy and of sections eighty-one A to eighty-one GG, inclusive, and they may appropriate money therefor. The planning board of a town, established under section seventy or corresponding provisions of earlier laws, may be authorized by vote of the town to act as park commissioners therein, and may be vested with all the powers and duties of park commissioners in towns.