See the guide to tree names for details of the use of subspecies, variety and cultivar names. Higher Levels of Classification Organisms are classified in a hierarchical scheme. The fundamental unit of classification is the species. Similar species are grouped into genera, genera into families and so on. All plants are eukaryotes.
All trees are in the Kingdom Plantae, the plants.
Recent research suggests that there should be more than 5 kingdoms, and that some organisms now in the plant kingdom should be moved into new kingdoms. We will update this paragraph as soon as that work is published. Division, Class, Subclass, Order . As more phylogenetic evidence becomes available, we expect considerable change in the assignment of species to these rankings. From a practical standpoint, these categories are currently of limited use to non-specialists.