A Federation world whose plant life is being destroyed by a botanical plague that only the mineral zenite from Ardana can stop, as told in The Cloud Minders.
Merak II never appears on screen, but its crisis drives the Original Series episode The Cloud Minders. A botanical plague is destroying all the vegetation on the planet, and unless it is stopped the world will become uninhabitable.
The only known remedy is zenite, a mineral mined on Ardana. The Enterprise is sent to collect a consignment, only to find that it has been seized by the Disruptors, rebellious Troglyte miners who are denied the comforts of the cloud city Stratos.
With only hours to spare, Kirk steps into Ardana's politics. He proves that raw zenite gives off a gas that dulls the mind, offers the miners filter masks and at last obtains the shipment from their leader Vanna, allowing the Enterprise to hurry to Merak II.