Section 31. Whoever hinders or obstructs the director or the said chief of the bureau of milk marketing, or any agent of either said person, in the discharge of any authority or duty imposed upon him by any provision of sections twenty-five to thirty, inclusive, whoever violates any provision of said sections, and whoever knowingly violates any provisions of the rules and regulations made thereunder, shall be punished by a fine of not less than twenty-five nor more than fifty dollars.