TAXONOMY AND PHYLOGENY OF THE ANGIOSPERMS
The Taxonomy and Phylogeny of the Angiosperms
Flowering Plants or Angiospermae [phylum] are the
dominant plants on earth at this time and have been for the last 100
million years. Traditionally they have been divided into two divisions
[classes] called Monocotyledons [one seed-leaf] and Dicotyledons [2 seed
leaves] based on the number of food stores contained in the seeds.
Monocots also have parallel venation while the others have complex netted
venation.
Apart from some other structural features of
importance, the classification of Angiosperms rests mostly upon the
structure of the flower, which is undoubtedly their most distinctive
feature. This is mainly because it is the flower which shows such
remarkable plasticity of form. This variability of the structure of the
flower in turn reflects the intimate adaptive association of flowers with
insects, whose evolution they have closely paralleled. Indeed, such a
phenomenon is often given as an example of co-evolution because of the
strength and longevity of the contact between the two groups, and the
diverse ways they have mutually influenced each other in structure and in
life-cycles.
“During the first 70 million years of
angiospermous evolution all the known flowers were radially
symmetrical. It is only in the Tertiary (66.4 to 1.6 million years
ago) during the late Paleocene and early Eocene (63.6 to 52 million
years ago) that the first evidence of bilaterally symmetrical flowers
is found. The evolution of bilateral flowers, as, for example, those
of the legumes and orchids, is an adaptation for specialized
pollinators such as social insects (bees) and some birds. The sterile
organs (sepals, petals) are modified to present a certain flower
orientation to the pollinator, enabling the pollinator to enter the
flower where the pollen organs and pollen-receptive tissue are
positioned to maximize effective pollination. During the early
Tertiary, the bilateral organization of floral organs coevolved with
animal behaviour independently at different times and in various
groups of angiosperms.” [Encyclopedia Brittanica]
There is within the Angiosperms a general trend from
primitive beetle and fly pollinated flowers towards more advanced
pollination by butterflies, moths and bees. Flowers pollinated by bees
show the greatest degree of nectar, scent, coloration diversity and
pollen production, as well as diverse specialisation of flower
structure. Primitive flowers tend to be trumpet or saucer shaped
[examples include Ranunculus, Hibiscus, Magnolia, Malva, Lavatera,
Cucurbita and Papaver], while more advanced flowers maybe
tubular [examples like Symphytum, Digitalis, Lamium, Erica, Datura and
Nicotiana], or zygomorphic. Examples of the latter include Orchis,
Antirrhiunum, Pisum, Lobelia, Viola, Gentiana, Wisteria, Mimulus,
Lonicera and Begonia. There are thus two directions of
adaptation: from polypetaly [many petals] to oligopetaly [fewer petals]
and from separate petals to sympetaly [fused petals]
However, simple flowers, as in Commelinidae
[Grasses, Sedges and Rushes and Euphorbiales] may not be
primitive, but derived from more complex forms in the Liliales.
This, their simplicity is secondary and a form of structural degeneracy
rather than a primitive feature.
Another important trend is away from single flowers
[like Papaver, Ranunculus, Nelumbo] towards multiple flowers in
racemes, spikes, whorls, and heads [eg. Lamium, Scrophularia,
Genitaiana, Borago, Laburnum, Plantago, Achillea, Salix]. This trend
reaches its height of perfection in the Dipsacales, the Umbelliferae
[Apiales] and above all in the Asterales. The Asteraceae
compound flowers are really a spike or whorl of flowers that has failed
to expand vertically and all the flowers are packed side by side instead
on a flat head [capitulum]. You can still trace the spiral patterns in Helianthus
heads, for example, where the whorl-like pattern of the raceme would
have unfolded if it had elongated vertically.
The inflorescence of daisies therefore give the
appearance of a raceme that has been compressed downwards forcing all
the individual flowers to become pressed tightly into a side by side
arangement on a flat head [e.g. Bellis, Achillea]. The dome shape
of the Dipsacales flowers is similar, like an early version of
this arrangement, but in the umbels of the Umbelliferae each
individual flower still retains its own separate stalk and bracts.
In recent years the old classification has been
drastically revised, mosty through the work of taxonomists Arthur
Cronquist in the USA and Armen Takhtajan in Russia. The Angiosperms are
now divided into two groups [classes] like before, but now called Magnoliopsida
[Magnolia like] and the Liliopsida [Lily like] – being
broadly the same as the older Dicots and Monocots respectively.
The Magnoliopsida are subdivided into 7
subclasses each divided into orders and the orders divided into
families. The Liliopsida are divided into only 4 sub-classes and
again into orders and families.
Liliopsida
Subclass Alismidae – 3 orders including frogbit,
water plantain, flowering rush, eel grass, pondweed
Subclass Liliidae – 5 orders including Orchids,
Ginger, Arrowroot, Iris, Lilies, Onion, daffodil
Subclass Commelinidae – 7 orders including grasses,
sedges, rushes, pineapple
Subclass Arecidae [Spadiciflorae] – 5 orders
including palms, duckweeds and Arum lilies
Magnoliopsida
Subclass Magnoliidae – 6 orders – magnolias and
relations
Subclass Ranunculidae – 5 orders – buttercups and
relations
Subclass Hamamelididae – 15 orders – witch-hazel and
relatives
Subclass Caryophyllidae – 5 orders – elder and
relations
Subclass Dillenidae 17 orders – Paeonies, violets and
myriad relations
Subclass Asteridae 8 orders – daisies and myriad
relations
Subclass Rosidae – 19 orders – Roses and myriad
relations
The links between the various groupings listed above
needs to be brought out in more detail in order to appreciate the shape
and contents of the group as a whole. The most primitive group are the Magnoliales,
from which all the others have evolved. They consist mostly of small
trees and woody shrubs with large upright flowers composed of many large
pale petals surrounding the many ovules and anthers forming a spike in
the centre. They are mostly beetle and fly pollinated.
In general terms we can see from the diagrams that
the entire group is derived from a primitive Angiosperm that gave rise
to the Magnoliales and the Nymphaeales. The Liliopsida
is divided into 4 subgroups. The first and most primitive includes all
aquatic plants, the Alismales, Hydrocharitales [derived from the Alismales]
and the Najadales. Both the Alismales and the Najadales
are derived from the Nymphaeales and those in turn from a
primitive Angiosperm or common ancestor of the Magnoliales.
Apart from a couple of minor families, the Magnoliales
then gave rise to 4 main branches:
[1] the woody Trochodendrales to Hamamelidales
branch, which forms the Hamamelidae and are almost all trees like
Walnut, beech, Birch, Elm, Oaks, Alder, etc;
[2] the largely herbaceous Ranunculales with
various offshoots including the Lotuses, Poppies and Caryophyllales –
the latter including the Cactaceae;
[3] The hugely adaptive and important Dilleniales,
a very large herbaceous group including 3 major divisions- the Violales,
Theales and Malvales, with numerous other important subgroups
and families;
[4] the advanced Saxifragales and their two
main offshoots, the Rosales/Fabales and the Asteridae.
Most of these are also herbaceous, though with some important woody
exceptions.
Liliopsida
These are derived from aquatic plants [the Nymphaeales
and Alismatales], and many still retain their fleshy,
hydrophilic, semi-amphibious preferences [e.g. reeds, rushes, irises and
sedges]. These plants often have fleshy bulbs and corms from which they
can regenerate vegetatively. The more simple and primitive are
colourful, showy and insect pollinated [like Irises and Lilies] while
many of the most advanced and specialised are wind pollinated, such as
the grasses, sedges and rushes. Many of the grasses are also xeromorphs
[dryness loving] which have been liberated from the hydrophilic habits
of their ancestors. Orchids are the most extreme example of advanced and
specialised liliopsids showing extreme insect adaptation with
zygomorphic scented flowers with remarkable specilaisation of shape and
coloration. n of shape and coloration.
Magnoliidae
These are mostly woody trees and shrubs and all are
regarded as the most primitive of the Angiosperms in general.
Trochodendrales to Hamameliidae
These are mostly woody shrubs and trees, wind
pollinated. Most of the important tree species are in this group,
including Elms, Oaks, Ash, Beeces, Birches, Walnuts and Alder.
Rafflesiales and Aristolochiales
These are mostly large, tropical herbs, specialised
and obscure plants that probably represent evolutionary cul-de-sacs.
Strangely shaped and coloured.
Ranunculales
These are mostly herbaceous plants, often with
actinomorphic, simple flowers, not especially fragrant or scented.
Predominantly with white or yellow flowers. Examples include Berberis,
Ranunculus, Aconitum, Anemone, Pulsatilla.
Caryophyllidae
Mostly actinomorphic and showy very colourful flowers
with numerous petals and numerous anthers; abundance of pollen; they
give a general impression of proliferation and abundance.
Dilleniidae
Woody or herbaceous; often fleshy plants with large
showy flowers and soft velvety leaves. Large trumpet-shaped flowers are
common throughout the group [e.g. Theales, Malvales, Passiflorales,
Cucurbitales, Primulales] or in tufts [e.g. Salicales,
Tamaricales] or more rarely small and inconspicuous, eg. Violales,
Ericales, Euphorbiales, Cruciferae and Begoniales.
As a group, these plants generally have very
colourful flowers, with an abundance of pollen and anthers but not often
very highly scented, fragranced or nectared flowers
Rosidae
Advanced and recent geologically. Woody or
herbaceous; numerous types; nectar and scented, colourful, showy, 5
patterned flowers; mostly actinomorphic but some zygomorphic [e.g. Leguminosae,
Linaceae, Hippocastanaceae, Balsaminaceae].
Asteridae
Mostly herbaceous; derived from Saxifrages, numerous
types; very advanced, recent and most evolved plants on earth; great
tendency to flowers in spikes, compounds, umbels and heads of densely
crowded flowers. Examples include Dipsacales, Gentianales,
Boraginaceae, Scrophulariales, Lamiales, Asterales. These plants
also often contain a lot of essential oils, aromatic compounds and thus
form medicinally important groups. Examples include members of the Asterales,
Lamiales, Dipsacales, Scrophulariales, Campanulales.
Terms
Actinomorphic radially symmetrical flower
e.g. daisy
Zygomorphic vertically or bilaterally symmetrical
flower e.g. snapdragon
Sources
An excellent outline of the seed plants can be found at:
http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookDiversity_6.html
another useful site:
http://phylogeny.arizona.edu/tree/eukaryotes/green_plants/embryophytes/angiosperms/agiosperms.html
good site:
http://dmoz.org/Science/Biology/Taxonomy/Taxonomy_Map/Plantae/Magnoliophyta/
and another:
http://www.ucmp.berkeley.edu/anthophyta/anthophytafr.html
and another excellent site, complete with illustrations:
http://biodiversity.bio.uno.edu/delta/angio/ms/angiosperms.html
another excellent illustrated site:
http://nature.snr.uvm.edu/www/mac/plant-id/angiosperms/angiosperms.html
the flowering plant gateway:
http://www.csdl.tamu.edu/FLORA/newgate/gateopen.htm
Some good basic Lecture Notes:
http://www.inform.umd.edu/PBIO/pb250/index.html
outline on phylogeny:
http://sif.plantbio.ohiou.edu/Angiophyte/Outline.htm
a good basic intro to the group:
http://buglady.clc.uc.edu/biology/bio106/angio.htm
Massive set of links for Angiosperms:
http://www.resnet.wm.edu/~mcmath/bio205/links/angiosperms.html
Books:
Cronquist, A. 1981. An integrated system of classification of flowering
plants. New York; Columbia Univ. Press.
Cronquist, A. 1988. The evolution and classification of flowering
plants, Bronx, New York; New York Botanical Garden.
Encyclopedia Brittanica, Angiosperms
Morrell, Peter, 1972, Flower Insect Relations, BSc project, summer
1972, unpublished dissertation, Universty of Leeds
Takhtajan, A. L. 1980. Outline of a classification of flowering plants
(Magnoliophyta), Bot. Rev. 46:225-359.
Takhtajan, A. L. 1987. Sistema Magnoliofitov (Systema
Magnoliophytorum), Acad. Sciences, USSR, Nauka, Leningrad. [In Russian.].
Takhtajan, A. 1991. Evolutionary Trends in Flowering Plants, New York;
Columbia Univ. Press.
Takhtajan, A.L. (1969). Flowering Plants, Origin and Dispersal,
(Translation from the Russian by C. Jeffrey). Oliver and Boyd, Edinburgh.