No tax shall be levied except in pursuance of law, and every law imposing a tax shall state distinctly the object of the same, to which only it shall be applied. Notwithstanding the foregoing or any other provisions of this constitution, the legislative assembly, in any law imposing a tax or taxes on, in respect to or measured by income, may define the income on, in respect to or by which such tax or taxes are imposed or measured or may define the tax itself by reference to any provision of the laws of the United States as the same may be or become effective at any time or from time to time, and may prescribe exceptions or modifications to any such provision.