Christen Raunkiaer and the Way Plants Grow

In 1903 the Danish botanist Christen Raunkiaer proposed a solution to the difficult botanical problem of comparing plant communities with entirely different compositions. Raunkiaer's idea was to categorize plants by the position of their perennating buds—the plant structure with which a plant survives periods of adverse conditions. Raunkiaer believed that flowering plants first appeared in the Tropics, and that they then spread from there into higher latitudes. In doing so, plants had to evolve strategies for surviving periods of cold or very dry weather. Different plants developed different strategies, and it was those strategies that he used as the basis of his categorization.

At the onset of the cold or dry season, the parts of a plant that are above ground level die back. Woody plants such as trees and shrubs lose their leaves and appear to be dead. Nonwoody plants wither and appear to die. The plants are not dead, however, and they store food in tubers, bulbs, rhizomes, or in the case of woody plants as buds. These are the perennating buds from which the plant will grow back when the weather improves. A tuber is a swollen stem or root that serves as an underground storage organ. A bulb is an underground storage organ consisting of a short, fat stem with roots at its base above which there are fleshy leaves surrounded by protective scales. A rhizome is a horizontal, creeping, underground stem from which roots and shoots emerge at intervals. All of these are perennating buds, and Raunkiaer identified five types, with some subdivisions, each of which offered a different degree of protection, depending on its distance from ground level. In order of the protection the position of the perennating buds affords, the Raunkiaer categories are as follows:

  • Phanerophytes are plants in which the perennating buds are above ground on shoots exposed to the air. This affords the least protection from harsh weather, and phanerophytes are found where cold or dry conditions occur infrequently. They are trees and shrubs and also epiphytes—plants that grow on the surfaces of other plants, using those plants only for support and not as a source of nutrients.
  • Chamaephytes are plants in which the perennating buds occur very close to the ground. This offers rather more protection. Hemicryptophytes are plants in which the perennating buds are at ground level.
  • Cryptophytes are plants in which the perennating buds are below the ground or water surface. There are three types of cryptophytes. Geophytes have perennating buds below the ground. Helophytes have perennating buds below the surface of a marsh. Hydrophytes have perennating buds below the surface of water.
  • Therophytes are plants that complete their life cycles rapidly when conditions are favorable, then die and survive as seed until favorable conditions return. These are annual plants that are also known as ephemeral plants or ephemerophytes.

Raunkiaer identified his categories by comparison with the world average strategy, which he called the normal spectrum, rather than the strategy found in the Tropics. The system he devised is still widely used. He also discovered that when the plant species growing in a specified area are counted and the numbers of the species are arranged in frequency classes, each comprising 20 percent of the total, plants were either very common or very rare. This came to be known as Raunkiaer's law. Raunkiaer believed that everything could be counted and understood statistically.

Christen Christensen was born on March 29, 1860, at Lynhe in western Jutland, Denmark, on a large farm called Raunkiaer. At that time it was customary in Scandinavia for people to have a first name and patronymic, but a person enrolling at a university had to provide a surname, so he took the name of the farm. Raunkiaer studied at the University of Copenhagen, graduating in botany in 1885. From 1885 to 1888 he taught at Borchs Botanical College, and in 1893 he obtained a position as a scientific assistant at the Copenhagen University botanical gardens and museum. In 1911 he succeeded Eugen Warming as professor of botany and director of the botanical gardens at Copenhagen, remaining in this post until 1923. He died in Copenhagen on March 11, 1938.