Section 31. The probate court may, upon the petition of a guardian or conservator setting forth a description of the real estate of his ward which he desires to lease, the reason why it is necessary or expedient to give a written lease thereof, and the length of the term, if, after notice and a hearing, it appears to be necessary or expedient, authorize such guardian or conservator to give a written lease of said real estate, and the decree of the court shall fix the term and the amount for which it may be leased.