The legislative authority of the state shall be vested in a Legislature consisting of one chamber. The people reserve for themselves the power to propose laws and amendments to the Constitution and to enact or reject the same at the polls, independent of the Legislature, which power shall be called the power of initiative. The people also reserve power at their own option to approve or reject at the polls any act, item, section, or part of any act passed by the Legislature, which power shall be called the power of referendum.