The Secretary of the Interior, in his administration of the national parks and national monuments, is authorized to maintain central warehouses at said parks and monuments, and appropriations made for the administration, protection, maintenance, and improvement of the said parks and monuments shall be available for the purchase of supplies and materials to be kept in said central warehouses for distribution at cost, including transportation and handling, to projects under specific appropriations, and transfers between the various appropriations made for the national parks and national monuments are authorized for the purpose of charging the cost of supplies and materials, including transportation and handling, drawn from central warehouses maintained under this authority to the particular appropriation benefited; and such supplies and materials as remain therein at the end of any fiscal year shall be continuously available for issuance during subsequent fiscal years and to be charged for by such transfers of funds between appropriations made for the administration, protection, maintenance, and improvement of said parks and monuments for the fiscal year then current without decreasing in any way the appropriations made for that fiscal year: Provided, That supplies and materials shall not be purchased solely for the purpose of increasing the value of storehouse stock beyond reasonable requirements for any current fiscal year.