3311.  Principal subject to depletion.--Where any part of the principal consists of property subject to depletion, such as leaseholds, patents, copyrights and royalty rights, and the trustee or tenant in possession is not under a duty to change the form of the investment of the principal, the full amount of rents, royalties or return from the property shall be income to the tenant; but where the trustee or tenant is under a duty, arising either by law or by the terms of the transaction by which the principal was established, to change the form of the investment, either at once or as soon as it may be done without loss, then the return from such property not in excess of five per centum per annum of its fair inventory value or in default thereof its market value at the time the principal was established, or at its cost where purchased later, shall be deemed income and the remainder principal.