The power of a real estate broker to give an option for a limited time, and to extend such time, is in the nature of a personal trust, so as to negative any implied power to appoint a sub-agent for whose services the principal will be liable. Sorenson v. Smith, 129 P. 737, 65 Or. 78, 51 L. R. A. (N. S.), 612, Ann. Cas. 1915 A, 1127; judg. aff. on re., 131 P. 1022, 65 Or. 78.