The articles of incorporation must be subscribed by five or more persons, who must be citizens of the United States, and acknowledged by each of them before some officer authorized by the laws of this state to take and certify acknowledgments of conveyances of real property situate within this state. Each subscriber may subscribe said articles personally or by an attorney-in-fact, thereunto duly authorized in writing signed by said subscriber and acknowledged before some officer authorized by the laws of this state to take and certify acknowledgments of conveyances of real property situated within this state. All such powers of attorney must be securely attached to said articles of incorporation, and be filed therewith, as hereinafter provided.