Section 18. Each agent or other person, except a grower, who sells nursery stock shall make application to the director for an agent's license, and shall file with him the names and addresses of all persons or nurseries from which he purchases his stock. On receipt of such application, the director shall issue an agent's license, valid for one year, in such form and with such provisions as the department prescribes. The license may be revoked at any time for failure to report the names and addresses of persons or nurseries from which stock is purchased, or for such other cause as may be deemed sufficient by the director and the commissioner. Each person except a grower who sells, or who takes or solicits orders for, nursery stock shall be regarded as a nursery agent for the purposes of sections eighteen to thirty-one, inclusive.