Physical conditions and the geographical distribution of living beings have varied over geological time. It is thus not possible to use current geographical and botanical classifications to study ancient botanical geography.

Groups based on the physiological properties of plants would perhaps be more stable. There are five basic categories, which more or less conform with the major geographical divisions. These include the megatherms, which need much heat and moisture and correspond to tropical plants. Their ancestors were more widespread. There are many species with a variety of botanical characters, but they are often lianas that are ligneous, with evergreen foliage. There are many epiphytes. The xerophiles need heat and dryness, and they are scattered around the equator and in hot dry regions. This category is made up of a large number of herbaceous perennials with roots capable of resisting dryness. There are many succulents, as well as thorny, narrow-leafed shrubs. The ancient distribution of xerophiles is still poorly understood, but today they grow throughout the Mediterranean region. The mesotherms need moderate heat and moisture. Many species are annual and biennial ligneous evergreens. Their geographical distribution has changed greatly. It is thus of obvious interest to have a term that is not based on geography. The group of mesotherms was mixed with that of megatherms, as it currently is with that of xerophiles. The microtherms are the group of plants from temperate climates, above all the herbaceous perennials, and deciduous ligneous types and conifers. Their current location was formerly occupied by the mesotherms and the megatherms. The hekistotherms are today found in the Arctic and Antarctic. A sixth group of plants, megistotherms, tolerate high heat, and, although rare today, were widespread in the carboniferous epoch.

The physiological groups are not congruent with the botanical and geographical groups. The main families are represented in all the groups, whereas comparable species from the same genus do not belong to the same group, and the anatomy of plant organs does not correspond to their physiology. This is in fact because of the role of heredity and selection. However, since modifications of physiology are rarer than those of form, physiology ultimately lasts longer on the scale of geological time. If as is shown by the experience of horticulturists, there is some concordance between physiological groups and geographical groups, the difficulty in defining the latter still leads to many anomalies. Thus, a classification of physiological groups is fully justified.