The tremendous diversity of plants makes any classification system of their life-forms discretionary; that is, each system is arbitrary in the sense that there are alternative, equally useful ways to categorize their life-forms. However, each system strives for objectivity to achieve a classification that is both informative and useful. Lifeform classification systems historically served one of two general utilitarian objectives, either to sort species into natural taxonomic systems or to correlate (and functionally relate) phenotypic features with the ecological settings and conditions in which particular species exist. The different goals of taxonomically versus ecologically based life-form classification systems have resulted in a plethora of terminology, some of which is now antiquated.
One of the earliest attempts to classify the life-forms of plants and animals was made by Aristotle (384-322 BC). Although contemporary historical sources indicate that his plant life-form classification system was based largely on the general appearance and growth habit of plants, all of Aristotle's writings on plants are lost. In contrast, the works of his pupil, Theophrastus (371-287 BC), have survived. Theophrastus' early botanical interest was in the description, classification, and medicinal use of plants. He was the first author to formally recognize what are called today trees, shrubs, under-shrubs, and herbs. He adopted no formal system of classification, but used size and the presence of wood in stems as his primary criteria for classifying plant life-forms.
Like many early classification systems that of Theophrastus and those that followed him ignored the algae and to a slightly lesser extent aquatic vascular plants. The first attempt to classify the algae and aquatic vascular plants was made by Carl Linnaeus (1707-78). He recognized three algal groups based exclusively on shape: (1) Conferva (thread-like algae), (2) Ulva (frond-like life-forms), and (3) Fucus (more robust, shrub-like algal forms). Although Linnaeus' objective was a natural classification system, his three algal groups were entirely artificial, because of extensive convergent, adaptive evolution among the various algal lineages, which are currently distinguished from each other taxonomically on the basis of biochemical, cytological, and untrastructural features rather than on the basis of body size, shape, or structure.
It was not until Alexander von Humboldt (1769-1859) that the concept of the plant life-form for the vascular plants became formalized. He recognized 15 life-form groups: (1) the banana form (e.g., Musa, Heliconia, and Strelitzia), (2) the palm form, (3) arborescent ferns, (4) the Arum form (e.g., Dracontium), (5) the conifer form (e.g., Taxus and Pinus), (6) all sharp-leaf forms (e.g., Stenocereus), (7) the tamarisk form (e.g., Mimosa and Gleditsia), (8) the mallow form (e.g., Hibiscus), (9) lianas (e.g., Vitis), (10) epiphytic orchids, (11) the cactus form, (12) the casuarina/Equisetum form, (13) grasses, (14) the mosses, and (15) lichens. Using these 15 groups, von Humboldt identified vegetational types, grouped them into physiognomic classes, and recorded their changes along latitudinal and elevational transects. However, he did not write extensively on the relationship between the environment and plant life-forms.
August H. R. Grisebach (1814-79) adopted von Humboldt's concepts and expanded his classification system to emphasize the dependence of plant life-forms on climate. Subsequent systems of classification in the nineteenth century adopted this Humboldtian perspective by emphasizing how life-form features are adapted to prevailing ecological conditions, particularly conditions during unfavorable periods of the growing season. Among the criteria used by many workers were (1) duration of shoot growth, (2) the length and direction of shoot development, (3) the position and structure of perennating (renewal) buds, (4) the size of the reproductively mature plant, (5) adaptations to conserve water, and (6) adaptations for plant-plant competitive interactions. The most widely used vascular plant life-form classification system during the twentieth century was formulated by Christen C. Raunkiaer (1860-1938). This system identifies life-forms based on a single criterion, that is, the extent to which renewal buds are protected from draught or extreme cold.
Regardless of the exact criteria employed for classification, the seminal work of von Humboldt and Grisebach codified the life-form concept in terms of the vegetative structure of plants and how it is adapted to its environment. Life-form classification systems consequently focus attention on convergent adaptive features. The life-form concept, therefore, has no bearing on the development, structure, or mode of development of reproductive structures, which serves as the principal criteria for classifying plants into species, genera, families, etc. Because taxonomically diverse species often occupy similar or identical ecological conditions, taxonomic classification systems and life-form classifications rarely sort plants into the same categories.
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