Gene banks is the mean of storing plant genetic material. This ex-situ system is based on three methods of storing the material: seed banks provide a controlled environment where seeds can be dried to low moisture content and stored at low temperature without losing their viability. Field gene banks such as arboreta, plantations, and botanical gardens are used for species that are difficult or impossible to store as seed, including many perennials, vegetatively propagated crops and tree species. In vitro methods which conserve plant parts, tissue or cells in a nutrient medium can be used to conserve species that do not readily produce seeds, or where the seeds cannot be dried without damaging them.