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life form[′līf ‚fȯrm]
the external appearance (habitus) of plants, reflecting their adaptation to environmental conditions. The term was proposed by the Danish botanist J. E. Warming (1884), who conceived it to be the form in which the vegetative body of the plant is in harmony with the external environment during the entire course of its life, from seed to death. The term also designates the unit of ecological classification of plants that embraces a group of plants with similar adaptive structures. This similarity is not necessarily connected with kinship and is often convergent (for example, cacti and some spurges, which form the “life form” of stemmed succulents). The life form depends mainly on the structures of aboveground and underground vegetative organs of plants and is connected with the rhythm of their development and their longevity. The life form develops in the course of evolution as a result of natural selection under various climatic, soil, and biocenotic conditions. The life forms of certain plant groups reflect their adaptation to spatial distribution and attachment to a territory and to the fullest use of the whole complex of conditions in their areas of distribution.
The concrete life form of each plant (tree, shrub, liana, cushion plant, prostrate form) changes during its ontogeny. One-year seedlings of spruce or oak do not yet have the forms of the evergreen or deciduous trees that are characteristic of their adult stages. The same species may have a different life form under different conditions; for example, many woody species (oak, beech, spruce, larch, Turkestan juniper) that form tall-trunked trees in the forest zone and in the forest belt of the mountains produce shrubby and prostrate forms at the northern and altitudinal boundaries of their distribution. Therefore, life form, as a unit of classification, is understood to mean the aggregate adult individuals under the life conditions normal for them.
The first physiognomic classification of the principal forms of plants according to their external appearance, which determines the landscape of their location, belongs to the German naturalist F. A. von Humboldt who, in 1806, distinguished 19 such forms. Predominantly physiognomic were the “basic forms” system of the Austrian botanist A. Korner (1863), the “plant forms” system of the German botanist A. Grisebach (1872), and the “life forms” system of the German taxonomist O. Drude (1913). However, the relationship of plant appearance to climate and the importance of biological characteristics was already emphasized in those systems. Classifications subsequently appeared that were based on special adaptive characteristics. Of these, the most widespread and popular was the classification of the Danish botanist C. Raunkiaer (1905, 1907), which was based on the position of growing points in relation to the surface of the soil under unfavorable conditions (in winter or during drought) and on the character of the protection of the points—that is, on characteristics that are readily observable. Raunkiaer distinguished the following five types of life forms: phanerophytes, in which the new growing points are high above the earth (trees, shrubs, treelike lianas, epiphytes); chamaephytes, or low plants in which the growing points are no higher than 20–30 cm above the earth and often winter under the snow (small shrubs, undershrubs, some perennial herbs); hemicryptophytes, or herbaceous perennials in which the points are at soil level and are protected by snow and fallen leaves; cryptophytes, in which the points are hidden under ground (rhizomal, tuberiferous, and bulbiferous geophytes) or under water (hydrophytes); and terophytes, or annuals that survive unfavorable periods in the form of seeds. Herbaceous plants are most often classified according to the system devised by the Soviet botanist G. N. Vysotskii (1915) and developed by L. I. Kazakevich (1922), in which the basis is taken to be the character of the underground organs and the capacity of the plants for vegetative propagation and seizure of territory: taproot plants (vegetative reproduction is ab-sent), cespitose, bulbiferous, and bulbotuberiferous plants (in these groups vegetative propagation is weakly expressed), and root-sucker plants (vegetative propagation is intense). V. R. Williams subdivided the life forms of cereals, according to the means of tillering and the position of the buds, into long-rhizomed, loosely bushed, and densely bushed.
The Soviet botanist I. G. Serebriakov in 1962 and 1964 proposed a classification of life forms in which the larger divisions (divisions and phyla) are distinguished according to the structure and longevity of the aboveground skeletal axes (trees with trunks that live dozens or hundreds of years, shrubs with axes that live 20–30 years, small shrubs that live five to ten years, and herbs with annual orthotropic shoots). Each type is further detailed according to a number of characteristics.
The study of life forms is important to the solution of a whole series of theoretical and practical questions. Thus, Raunkiaer used the percentage composition of life forms in the flora of a given region (“biological spectrum”) to characterize a climate (for example, the climate of the phanerophytes is the humid tropics, and that of hemicryptophytes is the northern temperate and cold zones). Geobotanists study life forms as the components of a phytocenosis that reflect ecological conditions. In the complex study of the edifiers (the principal species constituting a phytocenosis) of steppe vegetation, E. M. Lavrenko’s concept of the ecobiomorph is used; this concept differs somewhat from that of life forms and includes the physiological characteristics of the objects as well. The goal of comparative morphogenetic research is to elucidate the course of formation of life forms in the ontogeny, as well as in the phylogeny, of particular classificatory groups. The study of the changes in life form under the influence of various environmental factors is very important in work on the introduction of plants and is being conducted at a number of botanical gardens.
REFERENCESShmithüsen, I. Obshchaia geografiia rastitel’nosti. Moscow, 1966. (Translated from German.)
Serebriakov, I. G. “Zhiznennye formy vysshikh rastenii i ikh izuchenie.” InPolevaia geobotanika, [issue] 3. Moscow-Leningrad, 1964.
Serebriakov, I. G. Ekologicheskaia morfologiia rastenii. Moscow, 1962.
D. A. KRIVOLUTSKII