Section, acts Apr. 17, 1936, ch. 233, 49 Stat. 1226; May 18, 1937, ch. 223, 50 Stat. 181; May 17, 1938, ch. 236, 52 Stat. 393; June 16, 1939, ch. 208, 53 Stat. 834; June 18, 1940, ch. 396, 54 Stat. 474, which excepted Botanic Garden, in purchase of supplies and equipment, from provisions of section 5 of this title when aggregate amount did not exceed $50, was superseded by subsection (b) of section 6a of this title which was itself repealed by act Oct. 31, 1951, ch. 654, Sec. 1(107), 65 Stat. 705