Plant Taxonomy
Taxonomic Systems
• Predicitivity - Ideally our systems of
classification should allow us to place
similar species of plants together in the
same category
Two Types of Classification Schemes
• Artificial classification schemes - these
systems allow one to quickly categorize a
particular organism, usually so that it can be
quickly found in a book, a garden or an
herbarium
• Natural classification scheme - organize
together related groups of plants and have a
high degree of predictive power
Artificial Classification Scheme
Red Trillium
Natural Classification Scheme –
For Seed Plants
Ancient Classifications
Pennisetum – a grass - Poaceae
Carex – sedge meadow - Cyperaceae
Carex Flowers Cyperaceae
Juncus – rushes - Juncacaea
Vegetables from Brassica oleracea
Brassicaceae
Theophrastus 370-285 BCE
Dioscorides – 1st Century AD
Page from 15th century Arabic
edition of Dioscorides herbal
Leonhart Fuchs - 1542
Fuchs’s History of Plants - 1542
Gerard’s Herbal 1597
Cesalpino - De Plantis 1583
Gaspar Bauhin - 1623
1623 – first
use of
binomial
names
John Ray – Catalog 1688
John Ray’s Catalog of English Plants
Carolus Linneaus
Systema Naturae – first published 1735
Genera Plantarum – 1737 –
Linneaus’s copy with notes
Species Plantarum - 1753
Linneaus - Artificial Classification Scheme
A. P. de Candolle - 1813
De Candolle – Flore Francaise
Classification After Darwin
Ideal Classification Scheme
• Ideally we would construct a classification scheme which
progresses from primitive or ancestral traits to advanced
or derived traits
• Ideally each taxon would be monophyletic - arisen by
diversification from a single ancestor – the Plant Kingdom
– whether the Embrophytes (land plants) or Viridiplantae
(green algae plus land plants)
• In contrast polyphyletic groups have arisen from more
than one ancestor - the Protista
• A paraphyletic group is one in which all members possess
a single ancestor in common, but which does not constitute
all descendents of that ancestor - the Dicots
Developing Classification Systems
• In developing classification systems, we attempt to group
plants which share derived characteristics - presumably
these characters have only arisen once or at most a few
times - it is not always easy to tell what is a derived
character though
• Derived characters may arise independently in different
groups of plants through convergent evolution or parallel
evolution
• convergent - unrelated plants develop similar
characteristics due to common environment
• parallel - plants with common ancestor develop similar
characteristics even though the ancestor did not have that
characteristic
Adolf Engler - 1887
Elymus (rye) flowers - Poaceae
Charles Bessey - 1911
Magnolia grandifolia Magnoliaceae
Arthur J. Cronquist - 1981
The figure above (redrawn from Cronquist, 1988, fig. 6.1) depicts phyletic
relationships among subclasses of the Class Magnoliopsida, as envisioned by
Cronquist. This alignment features the Magnoliidae as including extant dicot elements
that carry the greatest number of archaic features (similar to the 'original' flowering
plant) and the Asteridae (Sunflowers and relatives) as the most 'derived' or specialized
element of the Class. The size of the 'balloons' is roughly proportional to the number
of species per subclass.
Basics of Characters
• A taxonomic character is any expressed attribute
of an organism that can be evaluated and that has
two or more discontinuous states or conditions for example the number of petals on a flower - can
be in 3's, 4's or 5's - thus distinct states and they
are discontinuous
• The taxonomic value of a characteristic is
increased if the biological significance of the
characteristic has been determined
Usefulness of Characters
• Different characteristics have differing degrees of utility in
terms of classification - a uniform characteristic may be
very good at demonstrating cohesion or relatedness at a
higher level of classification such as the family
• Conversely, some characters which have a great deal of
variability may be of little value in differentiating higher
orders of classification, such as family, but may be very
valuable in differentiating lower taxonomic groups such as
the genus or species
Goat Dandelion - Asteraceae
Buttercup - Ranunculaceae
Morphology
• Plants are highly plastic in their growth forms how tall they grow, their shape will vary
depending upon environment and growing
conditions
• However reproductive structures tend not to differ
in form from plant to plant of the same species they may differ in number, but form is
conservative - flowers, cones tend to be similar
within all members of a species - thus much plant
classification and identification is based upon
reproductive structures
Umbel – flower head in Apiaceae
White Pasque Flower Ranunculaceae
American
Licorice
Mint
Chickweed - Caryophyllaceae
Oak acorns - Fagaceae
Birch catkin Betulaceae