The limitations imposed by paragraph (7) of section 4(1) of the [1920 c. 67.] Government of Ireland Act 1920 precluding the Parliament of Northern Ireland from making laws in respect of trade with any place out of the part of Ireland within its jurisdiction shall not be construed so as to prevent that Parliament from making laws prohibiting or restricting the importation or removal into Northern Ireland of seeds of any weeds specified by or under an Act of that Parliament as being capable of causing injury to agriculture in Northern Ireland or of plants or parts of plants of any such weeds.