For the purpose of establishing, constructing and maintaining any botanic garden authorized by Section 43, the corporate authorities of any forest preserve district are authorized to levy annually a tax of not to exceed .015% of the value, as equalized or assessed by the Department of Revenue, upon all the taxable property in the district. This tax shall be levied and collected in the same manner as the general taxes of the forest preserve district and shall be in addition to the maximum of all other taxes and tax rates which the district is now or may hereafter be authorized to levy upon the aggregate valuation of all taxable property within the district and shall be exclusive of and in addition to the maximum amount and rate of taxes the district is now or may hereafter be authorized to levy under this Act or under any other law or laws which may limit the amount of tax which the district may levy for general purposes. The proceeds of the tax herein authorized shall be kept as a separate fund.