Plant life-form

Plant life-form schemes constitute a way of classifying plants alternatively to the ordinary species-genus-family scientific classification. In colloquial speech, plants may be classified as trees, shrubs, herbs (forbs and graminoids), etc. The scientific use of life-form schemes emphasizes plant function in the ecosystem and that the same function or "adaptedness" to the environment may be achieved in a number of ways, i.e. plant species that are closely related phylogenetically may have widely different life-form, for example "Adoxa" and "Sambucus" are from the same family, but the former is a small herbaceous plant and the latter is a shrub or tree. Conversely, unrelated species may share a life-form through convergent evolution.
While taxonomic classification is concerned with the production of natural classifications (being natural understood either in philosophical basis for pre-evolutionary thinking, or phylogenetically as non-polyphyletic), plant life form classifications uses other criteria than naturalness, like morphology, physiology and ecology.
"Life-form" and "growth-form" are essentially synonymous concepts, despite attempts to restrict the meaning of growth-form to types differing in shoot architecture. Most life form schemes are concerned with vascular plants only. "Plant construction types" may be used in a broader sense to encompass "planktophytes", "benthophytes" (mainly algae) and terrestrial plants.
A popular life-form scheme is the Raunkiær system.
History.
One of the earliest attempts to classify the life-forms of plants and animals was made by Aristotle, whose writings are lost. His pupil, Theophrastus, in "Historia Plantarum" (c. 350 BC), was the first who formally recognized plant habits: trees, shrubs and herbs.
Some earlier authors (e.g., Humboldt, 1806) did classify species according to physiognomy, but were explicit about the entities being merely plactical classes without any relation to plant function. A marked exception was A. P. de Candolle (1818) attempt to construct a natural system of botanical classification. His system was based on the height of the lignified stem and on plant longevity.
Eugenius Warming, in his account, is explicit about his Candollean legacy. Warming's first attempt in life-form classification was his work "Om Skudbygning, Overvintring og Foryngelse" (translated title "On shoot architecture, perennation and rejuvenation" - ) (1884). The classification was based on his meticulous observations while raising wild plants from seed in the Copenhagen Botanical Garden. Fourteen informal groups were recognized, based on longevity of the plant, power of vegetative propagation, duration of tillers, hypogeous or epigeous type of shoots, mode of wintering, and degree and mode of branching of rhizomes.
The term life-form was first coined by Warming ("livsform") in his 1895 book Plantesamfund, but was translated to "growthform" in the 1909 English version "Oecology of Plants".
Warming developed his life-form scheme further in his "On the life forms in the vegetable kingdom". He presented a hierarchic scheme, first dividing plants into heterotrophic and autotrophic, the latter group then into aquatic and terrestrial, the land plants into "muscoid", "lichenoid", "lianoid" and all other autonomous land plants, which again were divided into monocarpic and polycarpic. This system was incorporated into the English version of his 1895 book "Oecology of Plants". Warming continued 
working on plant life-forms and intended to develop his system further. However, due to high age and illness, he was able to publish a draft of his last system only
Following Warming's line of emphasizing functional characters, Oscar Drude devised a life-form scheme in his "Die Systematische und Geographische Anordnung der Phanerogamen" (1887). This was, however, a hybrid between physiognomic and functional classification schemes as it recognized monocots and dicots as groups. Drude later modified his scheme in "Deutschlands Pflanzengeographie" (1896), and this scheme was adopted by the influential American plant ecologists Frederic Clements and Roscoe Pound
Christen C. Raunkiær's classification (1904) recognized life-forms (first called "biological types") on the basis of plant adaptation to survive the unfavorable season, be it cold or dry, that is the position of buds with respect to the soil surface. In subsequent works, he showed the correspondence between gross climate and the relative abundance of his life-forms.
 (1931) reviewed the previous life-form schemes in 1931 and strongly criticized the attempt to include "epharmonic" characters, i.e., those that can change in response to the environment (see phenotypic plasticity). He tabulated six parallel ways of life-form classification:
Later authors have combined these or other types of unidimensional life-form schemes into more complex schemes, in which life-forms are defined as combinations of states of several characters. Examples are the schemes proposed by Pierre Dansereau and Stephan Halloy. These schemes approach the concept of plant functional type, which has recently replaced "life-form" in a narrow sense.
Classification systems.
Following, some relevant schemes.
Theophrastus (c. 350 BC).
Based on plant habit:
Humboldt (1806-1808).
Humboldt described 19 (originally 16) "Hauptformen", named mostly after some characteristic genus or family:
De Candolle (1818).
Based upon the duration of life and the height of the ligneous stem:
Raunkiær (1904-1907) plant life-forms.
Based on the place of the plant's growth-point (bud) during seasons with adverse conditions (cold seasons, dry seasons):
Clements (1920).
Vegetation-forms:
Du Rietz (1931).
Main life-forms ("Grundformen") system:
Growth-form system:
Ellenberg & Mueller-Dombois (1967).
Main groups of plant life forms:
Other categorizations.
Following, other morphological, ecological, physiological or economic categorizations of plants. According to the general appearance (habit):
According to leaf hardness, size and orientation in relation to sunlight:
According to the habitat:
According to the water content of the environment:
According to latitude (in vegetation classification):
According to climate (in vegetation classification):
According to altitude (in vegetation classification):
According to the loss of leaves (in vegetation classification):
According to the luminosity of the environment:
According to the mode of nutrition:
According to soil factors:
According to the capacity to avoid dehydration:
According to short-term fluctuations in water balance:
According to the range of drought/humidity tolerance:
According to longevity:
According to the type of photosynthesis:
According to origin:
According to biogeographic distribution:
According to invasiveness:
According to establishment time in an ecological succession:
According to human cultivation:
According to importance to humans (see ethnobotany):