The Secretary of Agriculture is authorized and directed to cause such shade, ornamental, fruit, and shelter-belt trees, shrubs, and vines as are adapted to the conditions and needs of the southern Great Plains area, comprised of those parts of the States of Colorado, Nebraska, Kansas, Texas, Oklahoma, and New Mexico lying west of the ninety-eighth meridian and east of the five thousand-foot contour line, to be propagated at one of the existing field stations of the Department of Agriculture in such area, and seedlings and cuttings and seeds of such trees, shrubs, and vines to be distributed free of charge under such regulations as he may prescribe for experimental and demonstration purposes within such area.