If the money derived from gifts, bequests, or legacies at any time exceeds the sum necessary for immediate expenditure on the parks and grounds, or if it is advisable to invest all or a part of such funds in an interest-bearing or productive investment, the board may invest such money in interest-bearing bonds of the Government of the United States, or the State of California, or in obligations issued pursuant to the provisions of the Federal Home Loan Bank Act, approved July 22, 1932, as now or hereafter amended, or in obligations issued pursuant to Title 4 of the National Housing Act, approved June 27, 1934, as now or hereafter amended. Thereafter the board may sell and dispose of these securities, or change the form of the investment.