It shall be the duty of the cemetery board and other body or commission having in charge the care and operation of cemeteries to invest all sums set aside from the sale of lots, and all sums of money received, and to care for the income of all money and property held in trust for the purposes designated herein: PROVIDED, HOWEVER, That all investments shall be made in municipal, county, school or state bonds, general obligation warrants of the city owning such cemetery, or in first mortgages on good and improved real estate.