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 BCE
Dioscorides – 1st Century AD
Page from 15th century Arabic edition of Dioscorides herbal
Leonhart Fuchs
Fuchs’s History of Plants
Gerard’s Herbal 1597
Cesalpino - De Plantis 1583
Gaspar Bauhin
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
Linneaus - Artificial Classification Scheme
A. P. de Candolle
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
Elymus (rye) flowers - Poaceae
Charles Bessey
Magnolia grandifolia - Magnoliaceae
Arthur J. Cronquist
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