The work of
a botanist consists in three different tasks: Nomenclature or how to name plants, Classifying or the study of the relationships between the taxa, identifying and producing flora and revisions to help naming of plants.
Classifications appeared well before Linnaeus but they were all artificial including Linnaeus 'sexual system' (1754). Theophrastus (370-285 B.C.) did classify plant according to their gross morphology: tree, herbs, shrubs......
Artificial: Non predictive classification. Classes are created to help identifying and group plants into smaller manageable groups. The groups are usually define by a single character.
THE LINNAEAN ORDERS AND THEIR MEANINGS
Flowers perfect, each with stamens and pistils.
Cl.1. Monandria: stamen 1
Cl. 2. Diandra: stamen 2
Cl.3. Triandra: stamen 3
Cl.4. Tetrandia: stamen 4
Cl.5. Pentandria: stamen 5
Cl.6. Hexandria: Stamen 6
Cl. 7. Heptandria: Stamen 7
Cl.8. Octandria: Stamen 8
Cl.9. Enneandria: Stamen 9
Cl.10. Decandria: Stamen 10
Cl.11. Dodecandria: Stamen 12-19
Cl.12. Icosandria: Stamen 20 or more, inserted on the calyx
Cl.13. Polyandra: Stamen 20 or more, inserted on the receptacle.
Cl.14. Didynimia: stamen 4; 2 long and 2 short
Cl.15. Tetradynamia: Stamen 6; 4 long and 2 short. Fl. cruciform
Cl.16. Monadelphia: Filaments united below in 1 set.
Cl.17. Diadelphia: Filaments united in 2 sets. Fl. papilionaceous.
Cl.18. Polyadelphia: Filaments united in 3 or more sets.
Cl.19. Syngenesia: Stamen 5, anthers united. Fl. compound.
Cl.20. Gynandria: Stamens and pistils combined
Stamens and pistils in different flowers.
Cl.21. Monoecia: stamen and pistils on the same individual
Cl.22. Dioecia: stamen and pistils on different individuals.
Cl.23. Polygamia: Fl. perfect and unisexual on the same or on different individuals.
Linnaeus 'sexual system' copied from the 10th edition of Systema Natura (1759).
Question: In which class, following Linnaeus sexual system, would Digitalis purpurea or Foxglove be placed ?
Natural classifications are predictive and delimit natural groups. They show the relationships between groups and respect the evolution theory. The groups are based on several combined characters.
Adanson, M. (1727-1806), Jussieu, A.L. (1748-1836) and Lamarck, J. (1744-1829) and other 18th century botanists created natural systems and their family definition and names are still in use nowadays.
It was only much later than the Darwin theory of evolution (1859) and genetic advances (Mendel 1900) enabled the first phylogenetic classifications to appear.
Question: In which family is Digitalis purpurea most commonly placed according to the latest natural classification ?
Circular representation of the phylogenetic classification of the dicotyledones redrawn from Sporne 'The morphology of Angiosperms' 1974
Cladogram of the orders of Angiosperms representated as the transection of an imaginary phylogenetic tree, adapted from Dahlgren.
Cladogram of the orders of angiosperms taken from the 1966 version of Takhtajan's scheme of classification
Classification is the production of a logical system of categories, each containing any numbers of organisms, which allows easier reference to its components (kind of organisms).
Convergence: the possession of similar characteristics in two or more groups without an intermediate common ancestor.
Divergence: The possession of different characteristics.
monophyeletic: a group composed of an ancestor and all its descendants: diagnosed by synapomorphies (shared derived characters)
Parallelism: the possession of similar characteristics by two or more taxa which do have a common ancestor.
paraphyletic: a group containing a common ancestor and some but not all descendants: diagnosed by symplesiomorphies.
many botanists wish to know something of the evolutionary history and relationships of a group of taxa; in other words they will want the classification to reflect the phylogeny (phylogenetic or evolutionary pathways) of the plants.
A taxon (pl. taxa) is any taxonomic grouping, such as a phyllum; a family or a species. It is a useful general term.
Taxonomy: study and description of the variation of organisms, the investigation of the causes and consequences of this variation, and the manipulation of the data obtained to produce a system of classification. Such a definition is wider than that sometimes given, and has intentionally been drawn up to coincide with the meaning of the term systematics. In fact the two terms are nowadays commonly used synonymously.
APG Angiosperm Phylogeny Group
The Angiosperm Phylogeny Group, or APG, refers to two international groups of systematic botanists who came together to try to establish a consensus view of the taxonomy of flowering plants that would reflect new knowledge in angiosperm relationships molecular systematics. (The Angiosperm Phylogeny Group. 1998 An Ordinal Classification for the Families of Flowering Plants. Annals of the Missouri Botanical Garden. )
Molecular systematics, is the use of the structure of molecules to gain information on an organism's evolutionary relationships. Previous classification being only based on morphological and anatomical characters.
The major difference of this new classification are:
- Not to use formal, scientific names above the level of order, but rather to have named clades, such as eudicots, monocots and rosids.
- To place a substantial number of taxa whose classification has traditionally been uncertain.
- To offer alternative classifications for some groups, in which for example a number of families can either be regarded as separate or can be merged into a single larger family. APG II refers to such groups as "bracketed" taxa.
As a result the Eudicots (true dicots) or tricolpates (refering to the tricolpate pollen grains) are created and previously Dicotyledones orders are placed into Basal Angiosperms and Magnoliids.
Phylogenetic tree of the Angiosperms according to the Angiosperms Phylogeny Group (http://www.mobot.org/MOBOT/research/APweb/ - April 2008)
Question: In which order, following the APG system, would Digitalis purpurea or Foxglove be placed ?
The hierarchy of taxonomic ranks
||Lili-opsida = Dicots, Monocots
||Rosa subgen. Rosa etc.
||Rosa sect. Rosa, Cassiorhodon etc. (sing. nouns) ; Rosa sect. Synstylae, Caninae etc. (plur. adject.)
||Rosa subsect. Pimpinellifoliae etc. (plur. adject.)
||Rosa ser. Stylosae etc.
||Rosa stylosa subsp. gallica
||Rosa stylosa var. glabra
||Rosa stylosa subvar. alba
||Rosa stylosa forma variegata
Be aware that the APG classification deliberately do not use formal, scientific names above the level of order, but rather has named clades, such as 'eudicots', 'monocots' and 'rosids'.
Advanced, Primitive and Evolved
One method of inferring phylogeny is to pinpoint primitive (i.e. ancient or least specialized) as opposed to advanced (i.e. recent or derived from the primitive) characters and to assign primitiveness to taxa which possess high proportions of the former.
Evolved can also be used to refer as a taxon with advanced characters.
|Chlorophyll/ no Chlorophyll
||Chlorophyll/ no Chlorophyll
|Bundles collateral/ scattered
||Bundles collateral/ scattered
|leaves evergreen/ deciduous
|Leaves opposite/ spiral
||Leaves spiral/ opposite
|Venation reticulate/ parallel
||Floral parts spiral/whorled
|Flowers polymerous/ oligomerous
||Flowers polymerous/ oligomerous
|Flowers bisexual/ unisexual
||Flowers bisexual/ unisexual
|Flowers monoecious/ dioecious
||Flowers monoecious/ dioecious
|Flowers petalous/ apetalous
||Flowers petalous/ apetalous
|Flowers actinomorphic/ zygomorphic
||Flowers actinomorphic/ zygomorphic
|Flowers polypetalous/ gamopetalous
||Flowers polypetalous/ gamopetalous
|Stamens many/ few
||Stamens many/ few
|Stamens separate/ fused
||Stamens separate/ fused
|Pollen powdery/ massed
|Flowers hypogynous/ epihynous
||Flowers hypogynous/ epihynous
|Gynoecium polycarpous/ oligocarpous
||Gynoecium polycarpous/ oligocarpous
|Gynoecium apocarpous/ syncarpous
||Gynoecium apocarpous/ syncarpous
||Fruits single/ aggregate
||Fruits capsule/ drupe or berry
|Seeds with/ without endosperm
||Seeds with/ without endosperm
Comparison of characters alleged to be primitive/ adavanced by Bessey 'The phylogenetic taxonomy of angiosperms' Ann.Mo.bot.Gdn 2, 109-164. 1915 and Hutchinson 'The families of Flowering Plants' Macmilan 1926.
Most of the characters used to study evolution are in relation with the flower, because they are less subject to natural selection. For example a plant growing in shady condition with lots of competition might evolved in a tree to reach more light.
Nevertheless a widely accepted evolution trend in the Angiosperms or flowering plants is the simplification of the flower morphology and the specialization in pollination mechanisms. Often we refer the Magnoliids as 'primitive' with there many undifferentiated tepals arranged in a spiral, their many unfused carpels and anthers also organized in a spiral. In comparison with an orchid having a whorled perianth, a fused zygomorphic corolla with 6 tepals, an inferior ovary and a targeted pollination mechanism.
A species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are often used, such as based on similarity of DNA or morphology. Presence of specific locally adapted traits may further subdivide species into subspecies.
This definition doesn' t always apply to plant where breeding between different species is common. This explains why species description and synonymies are so high in plants: there are 928 000 taxa entries in IPNI (International Plant Names Index) but there are thought to be only 13900 genera and 250 000 plant species on earth.
The concept of biological species is sometimes used. This concept is assessed on their morphological or structural distinctness (which is really subjective to the author).
You will often hear of a 'Lumper' or a 'Splitter'. A 'Lumper' will try to group taxa in quite heterogenous but robust group, the 'Splitter' will try to recognize and value any morphological difference observed.This can lead to great variations in the treatment of a same group.
IPNI (International Plant Names Index) has around 928 000 names and around 4000 are added every year. But the species number has dropped from 330 000 to around 240 000 in 20-30 years and is still diminishing. This fact shows well that the species concept is still misunderstood and problematic.
Chart showing the percentage fertility of hybrids between various taxa of Nigella from the Aegan region taken from Strid.
Mutations in the DNA and natural selection could be the main drive of evolution.
But other forces drive evolution like:
- Geographical isolation (Allopatric speciation) example with Platanus occidentalis from North America and Platanus orientalis form the Mediterranean region.
- Hybrids, Polyploidy... (Sympatric speciation)
- Apomictic species (produce seeds without fecondation) example with Rubus with 430species only in the UK or Taraxacum with 180 species in the UK..
- Inbreeding species.
Plants were first described by a common name which would vary in different languages or even areas. example: Foxglove.
They were then described using a latin descriptive sentence.
Linnaeus became famous by using the latin binomial name system. example: Digitalis purpurea L. Sp. Pl. 2: 621. 1753 [1 May 1753]
Where Digitalis represents the genus name and purpurea the species name, L. the abbreviation of the author of the description of the species, Sp. Pl. for Species Plantarum 2 page 621 published in 1753 the publication where this description can be found and the type specimen located
Nomenclature follows strict rules gathered under the International Botanical Code.
All those names can be found on IPNI or the International Plant Names Index
Identifying consists in giving a name to a plant specimen using a Flora, a botanical revision or simply by comparing with a herbarium specimen or photographs.
If no names match a description of a new species could be made.
Nowadays there are several Flora online and many revisions are available on journals that are also online.
Flora of North America
Flora of Pakistan
Flora of China
Flora of New Zealand
Flora of Australia
Flora of Chile
There are also Flora you ll find in libraries covering the rest of the world
Flora of the USSR
Flora of Tropical East Africa
Flora of British India.......
Judd, W.S.; Campbell, C.S.; Kellogg, E.A.; Stevens, P.F. 1999. 'Plants systematics, a phylogentic approach' Sinauer Associates Inc. Publ. , Sunderland, 464p.
Heywood, V.H. 1985 'Flowering Plants of the World' Equinox publ., Oxford, 336p.
Jeffrey, C. 1982. 'An introduction to plant taxonomy', 2nd ed. Cambridge Univ. Press, Cambridge. 154p.
Spichiger, R.; Savolainen, V.; Perret, M.; Figeat, M. 'Systematic Botany of Flowering Plants: A New Phylogenetic Approach to Angiosperms of the Temperate and Tropical Regions Published by Science Publishers, 2004
Stace, C.A. 1989. 'Plant Taxonomy and Biosystematics', 2nd ed. Cambridge Univ. Press, Cambridge.
Angiosperm Phylogeny Group http://www.mobot.org/MOBOT/research/APweb/
IPNI International Plant Names Index http://www.ipni.org/