They can contain collections of special types of plants , such as cacti , herb gardens , plants of certain parts of the world . Some of these collections need to be kept in greenhouses , such as tropical plants , alpine plants or other exotic plants . They may contain specialist plant collections such as cacti and succulent plants , herb gardens , plants from particular parts of the world , and so on ; there may be greenhouses , again with special collections such as tropical plants , alpine plants or other exotic plants .