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The early period in plant taxonomy began with the Greek naturalist
Theophrastus (c. 370-287 B.C.), who described some five hundred species
of plants in his History of Plants, the oldest botanical work in
existence. Theophrastus classified plants into four groups: herbs, undershrubs,
shrubs, and trees, the latter he considered the most highly developed.
Following the decline of the Greek and Roman civilizations, little botanical
progress was made until the emergence of the herbalists in the sixteenth
century. The herbalists printed elaborate and highly artistic illustrations
of plants on woodprints and metal plates, and strongly influenced botanical
illustrations for many years. From the ancients, the herbalists perpetuated
the doctrine of signatures, the notion that plants or plant parts that resembled
human body parts were created for the purpose of healing those body parts.
While the number of plants the herbalists described mounted, there became
a need for a simpler, more efficient method of classifying them. The great
Swedish botanist Carl Linnaeus (1707-1778), widely considered the
"Father of Taxonomy," classified plants solely on the number and arrangement
of its reproductive organs. This arrangement resulted in unnatural groupings,
but his system of hierarchical classification and his binomial nomenclature
proved useful, and are still the standard in taxonomy today. Predating Darwin,
Linnaeus believed in a Divine Order, and the idea of open-evolution with
no predetermined goal would have surely shocked him.
The modern period of taxonomy began to emerge when the French botanist
Bernard de Jussieu (1699-1777), and his son Antoine Laurent de
Jussieu (1748-1836), modified Linnaeus's unsatisfactory system in their
Genera plantarum secundum ordines naturales disposita (1789). This
is the first attempt at a natural classification of plants. Others who followed
the Jussieu's break with the Linaean system included the Scottish botanist
Robert Brown (1773-1858), French botanist Adolph Théodore
(1801-1876), British botanists George Bentham (1800-1884), Sir
Joseph Hooker (1817-1911), and the American botanist Asa Gray
(1810-1888).
These botanists worked in the wake of Darwin's revolutionary Origin of
Species (1859), and unlike the artificial classification systems of
Theophrastus and Linnaeus, their systems were natural systems, which were
attempts to classify plants according to their evolutionary sequence—now
called a phylogenetic system. Phylogenetics is the reconstruction
of the evolutionary history of that plant as closely as possible by examining
the relationship of its characteristics with those of similar plants, or
in modern times, by genetic analysis.
In 1909, a comprehensive world treatment of the plant kingdom, Die
natürlichen Pflanzenfamilien, was published by Adolf Engler
(1844-1930) and Karl Prantl (1849-1893) and others. The classification
scheme for flowering plants in this treatment is still followed in most herbaria
and in most manuals and floras, including Gray's Manual of Botany,
and Britton and Brown's An Illustrated Flora of the Northern United States
and Canada, the latter an indispensable botanical reference in my collection.
But the universe of living things is vast and complex. The Engler and Prantl
system had major shortcomings. Many plants tend toward a reduction of parts
during the course of evolution, and Engler and Prantl made the mistake of
equating "simple" with "primitive," thereby grossly misclassifying major
taxonomic groups.
Many of Engler and Prantl's misclassifications were amended when Charles
Edwin Bessey (1845-1915), in his "The Phylogenetic Taxonomy of Flowering
Plants" (Ann. Mo. Bot. Gard. 2: 108-164, 1915), propounded his "dicta," or
statements of the guiding principles he used to determine the degree of
primitiveness or evolutionary advancement of a plant group. These dicta reflected
Bessey's careful observations that floral parts of plants tend to evolve
toward aggregation, fusion, reduction, and loss of parts. Bessey's dicta
and new phylogenetic system represented a major shift in taxonomy and profoundly
influenced subsequent taxonomic thought.
Present-day taxonomy is as exciting and dynamic as ever. New molecular analyses
of plant DNA, as well as traditional methods of establishing phylogenetic
relationships, continue to fit the pieces in the grand evolutionary puzzle.
The works of Cronquist, Thorne, Takhtajan,
Dahlgren, and Reveal, and a host of other systematists have
made major contributions in plant phylogenetic taxonomy. Grouping organisms
into logical, coherent ranks in accordance with their evolutionary lineages
is a complex endeavor subject to much latitude in interpretation, thus giving
rise to a wide range of divergent views by taxonomists. Nonetheless, they
all share a common goal: to gather life's verdant hodge-podge of twigs and
branches scattered about the planet Earth and carefully assemble them into
a grand Tree of Life.
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