Proof of citizenship, under sections 21, 22 to 24, 26 to 28, 29, 30, 33 to 48, 50 to 52, 71 to 76 of this title and section 661 of title 43, may consist, in the case of an individual, of his own affidavit thereof; in the case of an association of persons unincorporated, of the affidavit of their authorized agent, made on his own knowledge, or upon information and belief; and in the case of a corporation organized under the laws of the United States, or of any State or Territory thereof, by the filing of a certified copy of their charter or certificate of incorporation.
(R.S. § 2321.)