The ancient Greeks developed a classification about 300 bce in which plants were grouped according to their general form—that is, as trees, shrubs, undershrubs, and vines. This classification was used for almost 1,000 years. Modern classifications of plants attempt to assign a plant to a particular taxon and establish relationships with other plants based on genetics, cytology, ecology, behaviour, and probable evolutionary lineages, in addition to gross morphology. Popular classifications, however, remain useful tools for studying the common stresses that the environment exerts on all plants and the general patterns of adaptation that are shown no matter how distantly plants are related.