Section, act Mar. 4, 1913, ch. 145, § 1 [part], 37 Stat. 854, provided that any class of nursery stock or of any other class of plants, fruits, vegetables, roots, bulbs, seeds, or other plant products of which the importation could be forbidden from any country or locality under the provisions of section 160 of this title could be imported for experimental or scientific purposes by the Department of Agriculture.