California has codified the so-called "equal dignity" rule for agents in Civil Code Section 2309 which provides:

"An oral authorization is sufficient for any purpose, except that an authority to enter into a contract required by law to be in writing can only be given by an instrument in writing."

The statute makes no mention of corporations or their officers, leaving open the question of its application to corporate officers.  The California Supreme Court held that Section 2309 does not apply to executive officers of a corporation:

"The executive officer of a corporation is something more than an agent. He is the representative 34 Cal. 2d 11, *15; 206 P.2d 847, **849; 1949 Cal. LEXIS 136, ***6 Page 7 of 8 of the corporation itself.'" (E. K. Wood Lumber Co. v. Moore Mill & Lumber Co., 97 F.2d 402, 408; see also, note, 1 A.L.R. 1132; 17 Cal.L.Rev. 70.)  This reasoning squares with sound principles and the necessities of modern business."

Jeppi v. Brockman Holding Co., 34 Cal. 2d 11, 17 (1949).

The Supreme Court's holding predates by several decades the enactment of Section 313 of the California Corporations Code which validates certain written documents when executed by specified officers.  See If You're Relying On The Signature Of Two Officers, You May Want To Think Again.