Iowa Const. art. III, 2nd div., § 6
The number of senators shall total not more than one-half the membership of the house of representatives. Senators shall be classified so that as nearly as possible one-half of the members of the senate shall be elected every two years.
Repealed and rewritten by Amendment 26 (1968)
See also this codified Iowa Constitution, Art. III, §34
Referred to in Iowa Code §42.4