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II. Introduction to Plant
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8. Plant Systematics and
Evolution
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C H A PT E R OU T L I N E
Early History of Classification 124
Carolus Linnaeus 124
How Plants are Named 126
Common Names 126
A CLOSER LOOK 8.1 The Language
of Flowers 128
Scientific Names 129
Taxonomic Hierarchy 130
Higher Taxa 130
What Is a Species? 131
A CLOSER LOOK 8.2 Saving
Species Through Systematics 133
The Influence of Darwin’s Theory
of Evolution 134
The Voyage of the HMS Beagle 134
Natural Selection 136
Phylocode 137
Chapter Summary 137
Review Questions 137
Further Reading 138
K EY C O N CE P T S
1.
2.
3.
4.
Scientific names are two-word names
called binomials that are internationally
recognized by the scientific community.
Carolus Linnaeus, an eighteenth-century
Swedish botanist, started the binomial
system and is therefore known as the
Father of Taxonomy.
With the publication in 1859 of On
the Origin of Species, Charles Darwin
proposed that species are not static
entities but are works in progress that
evolve in response to environmental
pressures.
Natural selection favors the survival
and reproduction of those individuals in
a species that possess traits that better
adapt them to a particular environment.
C H A P T E R
8
Plant Systematics
and Evolution
Fossils, such as this 160 million year old arucarian pine cone from
Argentina, were part of the evidence Charles Darwin used to formulate
his theory of evolution of species by means of natural selection.
123
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8. Plant Systematics and
Evolution
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Introduction to Plant Life: Botanical Principles
P
lant systematics is the branch of botany that is concerned with the naming, identification, evolution, and
classification (arrangement into groups with common
characteristics) of plants. In a strict sense, plant taxonomy
is the science of naming and classifying plants; however, in
this book the terms taxonomy and systematics are used interchangeably. The simplest form of classification is a system
based on need and use; early humans undoubtedly classified
plants into edible, poisonous, medicinal, and hallucinogenic
categories.
EARLY HISTORY OF
CLASSIFICATION
The earliest known formal classification was proposed by
the Greek naturalist Theophrastus (370–285 B.C.), who was a
student of Aristotle. In his botanical writings (Enquiries into
Plants and The Causes of Plants), he described and classified
approximately 500 species of plants into herbs, undershrubs,
shrubs, and trees. Because his influence extended through the
Middle Ages, he is regarded as the Father of Botany.
Two Roman naturalists who also had long-lasting impacts
on plant taxonomy were Pliny the Elder (A.D. 23–79) and
Dioscorides (first century A.D.). Both described medicinal
plants in their writings, and Dioscorides’s Materia Medica
remained the standard medical reference for 1,500 years.
From this period through the Middle Ages, little new botanical knowledge was added. Blind adherence to the Greek and
Roman classics prevailed, using manuscripts painstakingly
copied by hand in monasteries throughout Europe.
The revival of botany after its stagnation in the Middle
Ages began early in the Renaissance with the renewed interest in science and other fields of study. The invention of the
printing press in the middle of the fifteenth century allowed
botanical works to be more easily produced than ever before.
These richly illustrated books, known as herbals, dealt largely
with medicinal plants and their identification, collection, and
preparation. The renewal of interest in taxonomy can be traced
to the work of several herbalists; in fact, this period of botanical
history from the fifteenth through the seventeenth centuries is
known as the Age of Herbals. Another factor in the revival of
taxonomy was the global exploration by the Europeans during
this period, which led to the discoveries of thousands of new
plant species. In less than 100 years more plants were introduced to Europe than in the previous 2,000 years.
Carolus Linnaeus (fig. 8.1) was born in May 1707, in
southern Sweden, the son of a clergyman. He became interested in botany at a very young age through the influence of
his father, who was an avid gardener and amateur botanist. It
was expected that Linnaeus would also become a clergyman,
but in school he did not do well in theological subjects. He
did, however, excel in the natural sciences and entered the
University of Lund in 1727 to pursue studies in natural science and medicine. (At this time medical schools were the
centers of botanical study because physicians were expected
to know the plant sources of medicines in use.) After one year
he transferred to the University of Uppsala, the most prestigious university in Sweden. It was here that he published his
first botanical papers, which laid the foundations for his later
works in classification and plant sexuality. In 1732, he undertook a solo expedition to Lapland to catalog the natural history of this relatively unknown area. He later published Flora
Lapponica, a detailed description of the plants of this area.
Linnaeus received his medical degree in 1735 from the
University of Harderwijk in the Netherlands. Soon he came
under the patronage of George Clifford, a director of the Dutch
East India Company and one of the wealthiest men in Europe.
He served as Clifford’s personal physician and as curator
of his magnificent gardens, which housed specimens from
around the world. The 3 years he spent in the Netherlands
Carolus Linnaeus
By the beginning of the eighteenth century, it was common
to name plants using a polynomial (see fig. 8.4), which
included a single word name for the plant (today called the
genus name), followed by a lengthy list of descriptive terms,
all in Latin. This system had flaws. It was not standardized;
different polynomials existed for the same plant; and it was
cumbersome to remember some of the longer polynomials,
which could be a paragraph in length. This was the state of
taxonomy during the time of Linnaeus.
Figure 8.1 Statue of Carolus Linnaeus (1707–1778) holding
flowers of Indian blanket (Gaillardia pulchella) at the Linnaeus
Teaching Garden, Tulsa, OK.
Levetin−McMahon: Plants
and Society, Fifth Edition
II. Introduction to Plant
Life: Botanical Principles
8. Plant Systematics and
Evolution
CHAPTER 8
Figure 8.2 Frontispiece of Systema Naturae, one of the writings
of Linnaeus, in which he expounded on his ideas of classification.
were the most productive period in his life. During that time,
he completed several books and papers including Systema
Naturae, Fundamenta Botanica, and Genera Plantarum,
which expanded on his ideas of classification (fig. 8.2).
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Plant Systematics and Evolution
125
He returned to Sweden in 1738 and soon married Sara
Elisabeth Moraea. After setting up a medical practice in
Stockholm, he was appointed physician to the Swedish
Admiralty, specializing in the treatment of venereal diseases.
In 1741, he returned to the University of Uppsala as professor
of medicine and botany, a position he retained until retirement in 1775. Linnaeus was a popular teacher who attracted
students from all over Europe. Many of his students became
famous professors in their own right; others traveled to distant
lands collecting unknown specimens for Linnaeus to classify.
After suffering several strokes, he died in January 1778.
One of Linnaeus’s achievements was his sexual system of
plant classification, which did much to popularize the study of
botany. This system was based on the number, arrangement,
and length of stamens and thus divided flowering plants into
24 classes. Using this system, it was possible for anyone
to identify and name unknown plants. At the time, his language was risqué because he compared floral parts to human
sexuality, with stamens referred to as husbands and pistils∗
as wives; for example, “husband and wife have the same
bed” meant stamens and pistils in the same flower (fig. 8.3).
∗
Note that the older term of pistil, rather than carpel, has been used in this key.
Vegetable Kingdom
Key of the Sexual System
Marriages of plants
Florescence
Public marriages
Flowers visible to every one
In one bed
Husband and wife have the same bed
All the flowers hermaphrodite: stamens and pistils in the same flower
Without affinity
Husbands not related to each other
Stamens not joined together in any part
With equality
All the males of equal rank
Stamens have no determinate proportion of length
4. Four males
1. One male
2. Two males
5. Five males
3. Three males
6. Six males
7. Seven males
8. Eight males
9. Nine males
10. Ten males
11. Twelve males
12. Twenty males
13. Many males
With subordination
Some males above others
Two stamens are always lower than the others
14. Two powers
15. Four powers
(a)
With affinity
Husbands related to each other
Stamens cohere with each other, or with the pistil
16. One brotherhood
18. Many brotherhoods
17. Two brotherhoods
19. Confederate males
20. Feminine males
In two beds
Husband and wife have separate beds
Male flowers and female flowers in the same species
21. One house
22. Two houses
Clandestine marriages
Flowers scarce visible to the naked eye
24. Clandestine marriages
(b)
23. Polygamies
Figure 8.3 (a) Linnaeus’s sexual system related floral parts to human sexuality. (b) Hibiscus in the Mallow Family (Malvaceae) keys out to
feminine males because the stamens are attached to the style.
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Dr. Johann Siegesbeck, a contemporary of Linnaeus and
director of the botanical garden in St. Petersburg, was
shocked at the analogies and said such
loathsome harlotry as several males to one female
would not be permitted in the vegetable kingdom
by the Creator . . . . Who would have thought that
bluebells, lilies, and onions could be up to such
immorality?
Despite the opposition of some, the Linnaean sexual
method was easy to understand and simple for even the amateur botanist to use. This method, however, was an artificial
system grouping together clearly unrelated plants (in his system, cherries and cacti were grouped together); by the early
nineteenth century it was abandoned in favor of systems that
reflected natural relationships among plants.
Linnaeus’s greatest accomplishment was his adoption
and popularization of a binomial system of nomenclature.
When he described new plants, he conformed to the current
practice of using a polynomial. For convenience, however, he
began to add in the margin a single descriptive adjective that
would identify unequivocally a particular species (fig. 8.4).
He called this adjective the trivial name. This combination
later developed into the two-word scientific name, or binomial, described in the next section. Linnaeus used this system
consistently in Species Plantarum, published in 1753. This
work contains descriptions and names of 5,900 plants, all the
plants known to Linnaeus. The binomial system simplified
scientific names and was soon in wide use. In 1867, a group
of botanists at the International Botanical Congress in Paris
established rules governing plant nomenclature and classification. They established Species Plantarum as the starting
point for scientific names. Although the rules (formalized
in the International Code of Botanical Nomenclature) have
been modified over the years, the 1753 date is still valid,
and many names first proposed by Linnaeus are still in use
today.
Linnaeus’s contributions were not limited to botany since
the binomial system is used for all known organisms. He is
credited with naming approximately 12,000 plants and animals; for all his contributions to the field of taxonomy, he is
known as the Father of Taxonomy.
HOW PLANTS ARE NAMED
Names are useful because they impart some information
about a plant; it may be related to flower color, leaf shape, flavor, medicinal value, season of blooming, or location. Names
are necessary for communication; “if you know not the name,
knowledge of things is wasted.”
This discussion begins with a look at common names, or
what plants are called locally, and follows with an examination of internationally recognized scientific names.
Figure 8.4 A photograph from Species Plantarum illustrates the
beginning of the binomial system. Note the trivial names in the
margin next to the polynomial description for each species. The
trivial name was later designated as the species epithet, which,
together with the generic, forms the binomial.
Common Names
A close look at common names often reveals a keen sense of
observation, a fanciful imagination, or even a sense of humor:
trout lily, milkweed, Dutchman’s pipe, Texas bluebonnet,
ragged sailor, and old maid’s nightcap (table 8.1). Sometimes
the names even convey feelings or emotions (see A Closer
Look 8.1—The Language of Flowers).
Names have evolved over centuries but are sometimes
only used in a limited geographical area. Even short distances away, other common names may be used for the
same plant. Consider, for example, the many names for the
tree that many people call osage orange (Maclura pomifera)
(fig. 8.5a): bodeck, bodoch, bois d’arc, bow-wood, osage
apple tree, hedge, hedge apple, hedge osage, hedge-plant
osage, horse apple, mock orange, orange-like maclura, osage
apple, and wild orange.
Levetin−McMahon: Plants
and Society, Fifth Edition
II. Introduction to Plant
Life: Botanical Principles
8. Plant Systematics and
Evolution
CHAPTER 8
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Table 8.1 Some Common Names
and Their Meanings
Names in Commemoration
Douglas-fir
David Douglas, plant collector
(1798–1834)
Camellia
Georg Josef Kamel, pharmacist
(1661–1706)
Gerber daisy
Traugott Gerber, German explorer
(?–1743)
Freesia
Friedrich H. T. Freese, German
physician (?–1876)
Names That Describe Physical Qualities
Dusty miller
White woolly leaves
Dutchman’s breeches
Shape of flower
Goldenrod
Shape and color of inflorescence
Indian pipe
Shape of flower with stem
Cattail
Inflorescence of carpellate flowers
Lady’s slipper
Shape of this orchid’s flower
Milkweed
Milky juice when plant is cut
Skunk cabbage
Fetid odor of inflorescence
Cheeses
Fruit resembles a round head of cheese
Smoke tree
Plumelike pedicels
Shagbark hickory
Shedding bark
Redbud
Color of flower buds
Quaking aspen
Rustling leaves
Bluebell
Color and shape of flower
Crape myrtle
Wavy edges of petals
(a)
Scientific Names That Have Become Common Names
Hydrangea
Abelia
Vanilla
Narcissus
Coreopsis
Gladiolus
Names That Indicate Use
Daisy fleabane
Gets rid of fleas
Boneset
A tonic from this plant can heal bones
Feverwort
Medicinal property to reduce fever
Kentucky coffee tree
Seeds roasted for coffee substitute
Belladonna
Juice used to beautify by producing
pallid skin and dilated, mysterious eyes
Names That Indicate Origin, Location, or Season
Pacific yew
Grows along northern Pacific coast
Spring beauty
One of the first flowers of spring
Marshmallow
Found in wet, marshy habitat
Daylily
Flowers last only a day
Four-o’clock
Flowers open in late afternoon
Japanese honeysuckle
Country of origin is Japan
(b)
Figure 8.5 Mock orange is a common name shared by (a) the
tree Maclura pomifera and (b) the shrub Philadelphus lewisii—two
entirely different species of plants.
Levetin−McMahon: Plants
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8. Plant Systematics and
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A CLOSER LOOK 8.1
The Language of Flowers
Through traditions, some flowers became symbolic of
certain emotions and feelings. This was sometimes even
reflected in their common names; two straightforward
examples are forget-me-not flowers, which conveyed the
sentiment “remember me,” and bachelor’s button, which
indicated the single status of the wearer. This symbolism
reached its peak during the Victorian era, when almost every
flower and plant had a special meaning. In Victorian times, it
was possible to construct a bouquet of flowers that imparted
a whole message (box fig. 8.1). A Victorian suitor might send
a bouquet of jonquils, white roses, and ferns to his intended,
which indicated that he desired a return of affection, he was
worthy of her love, and he was fascinated by her. This “language” became so popular that dictionaries were printed to
interpret floral meanings. One of the most popular dictionaries was the Language of Flowers (1884) by Kate Greenaway,
a well-known illustrator of children’s books. Following is a
small sampling of some common flowers and plants and what
they symbolized:
Amaryllis: pride
Apple blossom: preference
Bachelor’s buttons: celibacy
Bluebell: constancy
Buttercup: ingratitude
Yellow chrysanthemum: slighted love
Daffodil: regard
Daisy: innocence
Dogwood: durability
Elm: dignity
Goldenrod: caution
Holly: foresight
Honeysuckle: generous and devoted affection
Ivy: fidelity
Lavender: distrust
Lichen: dejection
Lily of the valley: return of happiness
Live oak: liberty
Magnolia: love of nature
Marigold: grief
Mock orange: counterfeit
Oak leaves: bravery
Palm: victory
Pansy: thoughts
Spring crocus: youthful gladness
128
Box Figure 8.1 Flowers convey a message all their own.
Dwarf sunflower: adoration
Tall sunflower: haughtiness
Yellow tulip: hopeless love
Blue violet: faithfulness
Wild grape: charity
Zinnia: thought of absent friends
Even today, several plants have well-known symbolic
meanings. Red roses convey passionate love; a four-leaf clover means luck; orange blossoms symbolize weddings; and
an olive branch indicates peace. Floral colors can also communicate feelings, with red indicating passion; blue, security;
yellow, cheer; white, sympathy; and orange, friendship. With
some thought, it is possible to find the right flower and color
to express the exact message.
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On the other hand, different plants may share the same
common name. Although mock orange is one of the common
names for osage orange, the name mock orange is usually
associated with a completely unrelated group of flowering
shrubs (Philadelphus spp.) (fig. 8.5b). These examples point
out the difficulties with common names; one plant may be
known by several different names, and the same name may
apply to several different plants. The need to have one universally accepted name is fulfilled with scientific names.
Scientific Names
Each kind of organism is known as a species, and similar species form a group called a genus (pl., genera). Each species
has a scientific name in Latin that consists of two elements;
the first is the genus and the second is the specific epithet.
Such a name is a binomial, literally two names, and is always
italicized or underlined; for example, Maclura pomifera is
the scientific name for osage orange. A rough analogy of the
binomial concept can be seen in a list of names in a telephone
directory, where the surname “Smith” (listed first) represents
the genus and the first names (John, Frank, and Mary) define
particular species within the genus.
In the binomial, the first name is a noun and is capitalized; the second, written in lower case, is usually an adjective.
After the first mention of a binomial, the genus name can be
abbreviated to its first letter, as in M. pomifera, but the specific epithet can never be used alone. The genus name, however, can be used alone, especially when referring to several
species within a genus; for example, Philadelphus refers to
over 50 species of mock orange. The specific epithet can be
replaced by an abbreviation for species, “sp.” (or “spp.” plural), when the name of the species is unknown or unnecessary
for the discussion. In the previous example, Philadelphus sp.
refers to one species of mock orange whereas Philadelphus
spp. refers to more than one species.
Scientific names may be just as descriptive as common
names, and translation of the Latin (or latinized Greek) is
informative (table 8.2). Sometimes either the genus name or
the specific epithet is commemorative, derived from the name
of a botanist or other scientist. Some specific epithets are
frequently used with more than one genus, and knowledge of
their meanings will provide some insight into scientific names
encountered later in this text (table 8.3).
A complete scientific name also includes the name or
names of the author or authors (often abbreviated) who first
described the species or placed it in a particular genus. For
example, the complete scientific name for corn is Zea mays
L.; the “L” indicates that Linnaeus named this species. On the
other hand, the complete name for osage orange is Maclura
pomifera (Raf.) Schneid. This author citation indicates that
Rafinesque-Schmaltz first described the species, giving it
the specific epithet pomifera, but Schneider later put it in the
genus Maclura. In this text, the author citations are omitted
for simplicity.
Plant Systematics and Evolution
129
Table 8.2
Genus Names and Their Meanings
Names in Commemoration
Begonia
Michel Begon, patron of botany (1638–1710)
Forsythia
William Forsyth, gardener at Kensington
Palace (1737–1804)
Bougainvillea
Louis Antoine de Bougainville, explorer and
scientist (1729–1811)
Fuchsia
Leonhard Fuchs, German physician and
herbalist (1501–1566)
Zinnia
Johann Gottfried Zinn (1727–1759)
Wisteria
Caspar Wistar, American professor of plant
anatomy (1761–1818)
Names That Describe Physical Qualities
Myriophyllum
Finely divided leaves
Chlorophytum
Green plant
Lunaria
Moon, refers to appearance of pods
Helianthus
Sunflower
Zebrina
Zebra, refers to striped leaves
Trillium
Floral parts in threes
Tetrastigma
Four-lobed stigma
Ribes
Acid tasting; refers to fruit
Polygonum
Many knees; refers to jointed stems
Zanthoxylum
Yellow wood
Sagittaria
Arrow; refers to arrowhead leaves
Names from Aboriginal or Classic Origins
Avena
Oats (Latin)
Triticum
Wheat (Latin)
Allium
Garlic (Greek)
Catalpa
Catalpa (North American Indian)
Vitis
Grape vine (Latin)
Ulmus
Elm (Latin)
Pinus
Pine (Latin)
Solidago
Make whole or strengthen
Angelica
Angelic medicinal properties
Cimcifuga
Repel bugs
Saponaria
Soap; refers to soap that can be made from
the plant
Pulmonaria
Lung; used to treat infections of the lung
Potentilla
Powerful; refers to its potent medicinal
properties
Names That Indicate Use
Names That Indicate Location
Elodea
Grows in marshes
Petrocoptis
Break rock; refers to habit of growing in rock
crevices
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Table 8.3 Common Scientific
Epithets and Their Meanings
A scientific name is unique, referring to only one species and universally accepted among scientists. It is the key
to unlocking the door to the accumulated knowledge about
a plant. Imagine the confusion if only common names were
used and a reference was made to mock orange. Would
this reference allude to Maclura pomifera or to a species of
Philadelphus?
acidosus, -a, -um
Sour
aestivus, -a, -um
Developing in summer
albus, -a, -um
White
alpinus, -a, -um
Alpine
annus, -a, -um
Annual
arabicus, -a, -um
Of Arabia
arboreus, -a, -um
Treelike
arvensis, -a, -um
Of the field
biennis, -a, -um
Biennial
campester, -tris, -tre
Of the pasture
canadensis, -is, -e
From Canada
carolinianus, -a, -um
From the Carolinas
chinensis, -is, -e
From China
coccineus, -a, -um
Scarlet
deliciosus, -a, -um
Delicious
dentatus, -a, -um
Having teeth
Higher Taxa
domesticus, -a, -um
Domesticated
edulis, -is, -e
Edible
esculentus, -a, -um
Tasty
europaeus, -a, -um
From Europe
fetidus, -a, -um
Bad smelling
floridus, -a, -um
Flowery
foliatus, -a, -um
Leafy
hirsutus, -a, -um
Hairy
japonicus, -a, -um
From Japan
lacteus, -a, -um
Milky white
littoralis, -is, -e
Growing by the shore
luteus, -a, -um
Yellow
mellitus, -a, -um
Honey-sweet
niger, -ra, -um
Black
occidentalis, -is, -e
Western
odoratus, -a, -um
Fragrant
officinalis, -is, -e
Used medicinally
robustus, -a, -um
Hardy
ruber, -ra, -rum
Red
saccharinus, -a, -um
Sugary
sativus, -a, -um
Cultivated
silvaticus, -a, -um
Of the woods
sinensis, -is, -e
Chinese
speciosus, -a, -um
Showy
tinctorius, -a, -um
Used for dyeing
utilis, -is, -e
Useful
vernalis, -is, -e
Spring flowering
virginianus, -a, -um
From Virginia
Species that have many characteristics in common are
grouped into a genus, one of the oldest concepts in taxonomy
(fig. 8.6). In almost every society, the concept of genus has
developed in colloquial language; in English the words oak,
maple, pine, lily, and rose represent distinct genera. These
intuitive groupings reflect natural relationships based on
shared vegetative and reproductive characteristics. Many of
the scientific names of genera are directly taken from the
ancient Greek and Roman common names for these genera
(Quercus, old Latin word for oak).
The next higher category, or taxon (pl., taxa), above
the rank of genus is the family. Families are composed of
related genera that again (as in a genus) share combinations
of morphological traits. In the angiosperms, floral and fruit
features are often used to characterize a family. Ideally, the
family represents a natural group with a common evolutionary lineage; some families may be very small while others
are very large, but still cohesive, groups. A few common
angiosperm families that have special economic importance
are listed in Table 8.4. According to the International Code
of Botanical Nomenclature, each family is assigned one
name, which is always capitalized and ends in the suffix
-aceae. The old established names of several well-known
families present exceptions to this rule. Both the traditional and standardized names are used for these families
(table 8.5). The taxa above the rank of family and their
appropriate endings are presented in Table 8.6. The higher
the taxonomic category, the more inclusive the grouping
(fig. 8.7). Families are grouped into orders, orders into
classes, classes into divisions (phyla)*, and divisions into
kingdoms. A domain is above the kingdom level and is the
vulgaris, -is, -e
Common
TAXONOMIC HIERARCHY
In addition to genus and species, other taxonomic categories
exist to conveniently group related organisms. As pointed out,
Linnaeus used an artificial system; however, today scientists
use a phylogenetic system to group plants. In a phylogenetic
system, information is gathered from morphology, anatomy,
cell structure, biochemistry, genetics, and the fossil record to
determine evolutionary relationships and, therefore, natural
groupings among plants.
*Either division or phylum (sing.; phyla, plural) may be used to indicate the taxonomic
rank that is composed of a group of related classes. Traditionally, division has been the
term preferred by botanists and will be used throughout this textbook.
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(a) Quercus phellos
(willow oak)
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Plant Systematics and Evolution
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(b) Quercus rubra
(red oak)
Figure 8.6 A genus is a group of species that share many characteristics in common. Although willow oak (Quercus phellos) and red oak
(Quercus rubra) are clearly distinct species, they are both recognizable as belonging to the oak (Quercus) genus by the presence of acorns.
most inclusive taxonomic category. The complete classification of a familiar species is also illustrated in Table 8.6.
In addition to the categories already described, biologists
also recognize intermediate categories with the “sub”
for any rank; for example, divisions may be divided into
subdivisions, and species may be divided into subspecies
(varieties and forms are also categories below the rank of
species).
Although the International Code of Botanical
Nomenclature has rules that govern the assignment of names
and define the taxonomic hierarchy, it does not set forth any
particular classification system. As a result, there are several
organizational schemes that have supporters. These systems
differ in the numbers of classes, divisions, kingdoms, and even
domains and how they are related to one another. Presently
most biologists use a three-domain, six-kingdom system,
which will be described fully in Chapter 9. There is general
agreement about the use of a three-domain, six-kingdom system, but biologists still debate the definition of a species.
What Is a Species?
As indicated previously, each kind of organism is known
as a species. Although this intuitive definition, based on
morphological similarities, works fairly well in many circumstances, it is limited; scientists have given much thought
to the biological basis of a species. Many accept the biological species concept first proposed by Ernst Mayr in 1942,
which defines a species as “a group of interbreeding populations reproductively isolated from any other such group of
populations.”
This definition presents problems when defining plant
species. Many closely related plant species that are distinct
morphologically are, in fact, able to interbreed; this is true
for many species of oaks and sycamores. By contrast, a
single plant species may have diploid and polyploid (more
than the diploid number of chromosomes) individuals that
may be reproductively isolated from each other. It is estimated that as many as 40% of flowering plants may be
polyploids, with the evening primrose group a thoroughly
studied example; an even higher percentage of polyploid
species occurs in ferns. Because of these limitations, alternatives to the biological species concept have been suggested. The ecological species concept recognizes a species
through its role in the biological community as defined by
the set of unique adaptations within a particular species to
its environment. The availablity of molecular sequence data
for nucleic acids and proteins had led to the development
of the genealogical species concept. Proponents utilize the
distinct genetic history of organisms to differentiate species.
Despite the lack of an all-inclusive botanical definition, the
concept of “species” facilitates the naming, describing, and
classifying of plants in a uniform manner. An inventory of
the world’s species is the first step in preserving biodiversity, as discussed in A Closer Look 8.2—Saving Species
through Systematics.
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Table 8.4
Economically Important Angiosperm Families
Scientific Family Name
Common Family Name
Economic Importance
Aceraceae
Maple
Lumber (ash, maple), maple sugar
Apiaceae
Carrot
Edibles (carrot, celery), herbs (dill), poisonous (poison hemlock)
Arecaceae
Palm
Edibles (coconut), fiber oils and waxes, furniture (rattan)
Asteraceae
Sunflower
Edibles (lettuce), oils (sunflower oil), ornamentals (daisy)
Brassicaceae
Mustard
Edibles (cabbage, broccoli)
Cactaceae
Cactus
Ornamentals, psychoactive plants (peyote)
Cannabaceae
Hemp
Psychoactive (marijuana), fiber plants
Cucurbitaceae
Gourd
Edibles (melons, squashes)
Euphorbiaceae
Spurge
Rubber, medicinals (castor oil), edibles (cassava), ornamentals (poinsettia)
Fabaceae
Bean
Edibles (beans, peas), oil, dyes, forage, ornamentals
Fagaceae
Beech
Lumber (oak), dyes (tannins), ornamentals
Iridaceae
Iris
Ornamentals
Juglandaceae
Walnut
Lumber, edibles (walnut, pecan)
Lamiaceae
Mint
Aromatic herbs (sage, basil)
Lauraceae
Laurel
Aromatic oils (bay leaves), lumber
Liliaceae
Lily
Ornamentals, poisonous plants
Magnoliaceae
Magnolia
Ornamentals, lumber
Malvaceae
Mallow
Fiber (cotton), seed oil, edibles (okra), ornamentals
Musaceae
Banana
Edibles (bananas), fibers
Myrticaceae
Myrtle
Timber, medicinals (eucalyptus), spices (cloves)
Oleaceae
Olive
Lumber (ash), edible oil and fruits (olive)
Orchidaceae
Orchid
Ornamentals, spice (vanilla)
Papaveraceae
Poppy
Medicinal and psychoactive plants (opium poppy)
Piperaceae
Pepper
Black pepper, houseplants
Poaceae
Grass
Cereals, forage, ornamentals
Ranunculaceae
Buttercup
Ornamentals, medicinal and poisonous plants
Rosaceae
Rose
Fruits (apple, cherry), ornamentals (roses)
Rubiaceae
Coffee
Beverage (coffee), medicinals (quinine)
Rutaceae
Citrus
Edible fruits (orange, lemon)
Salicaceae
Willow
Ornamentals, furniture (wicker), medicines (aspirin)
Solanaceae
Nightshade
Edible (tomato, potato), psychoactive, poisonous (tobacco, mandrake)
Theaceae
Tea
Beverage (tea)
Vitaceae
Grape
Fruits (grapes), wine
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A CLOSER LOOK 8.2
Saving Species through Systematics
Earth is blessed with a tremendous variety of living organisms. About 1.4 million living microbes, fungi, plants, and
animals have been identified by systematics. The number of
yet undescribed species is much greater, with estimates ranging between 10 million and 100 million. Biodiversity is an
inventory of the number and variety of organisms that inhabit
Earth. We are currently in the midst of a biodiversity crisis;
the variety of living species is declining owing to an accelerated extinction rate. Human activities are responsible for this
terrible loss. Over 6.6 billion people at present inhabit Earth,
and this number is expected to increase to over 9 billion in
the next 50 years. Population pressures cause natural areas
to be cleared for agriculture or expanding urbanization. More
people also results in more pollution that fouls the land, sea,
and air. All of these human-induced changes translate into a
death toll upon the world’s biodiversity.
Consider the tropical rain forests of the world. These
forests are some of the most biologically diverse areas on the
planet, home to approximately 70% of the world’s species.
Unfortunately, these forests have been subjected to massive
destruction. Scientists have calculated that species loss in the
tropical rain forests is currently 1% to 5% per decade and will
increase to 2% to 8% by the year 2015. This rate translates
to an average loss of 9,000 species per year, or a heartbreaking 225,000 extinctions between 1990 and 2015.
Why should we care about biodiversity? Biodiversity
is the basis for the necessary essentials to human existence:
food, fiber, fuel, and shelter. Of the estimated more than
250,000 species of angiosperms, nearly 20,000 have been
used at one time or another as food for humans. Advances in
agriculture are dependent upon the interaction between systematics and biodiversity. Since the 1960s, world crop yields
have increased two- to four-fold. Part of this increase is due
to the creation of improved crop varieties through breeding
programs and more recently through genetic engineering.
Locating and identifying relatives of crop species have been
of critical importance to agricultural research in breeding for
desirable characteristics. With the advent of genetic engineering, nearly any plant species is a potential source of genes
for transfer to agricultural crops. Ironically, the conversion
of native ecosystems to agricultural lands in an attempt to
accommodate the food demands of an exponentially growing human population may eliminate the very organisms on
which agriculture depends for its future. Fertile soil, obviously essential for the vitality of agricultural crops, is also a
by-product of biodiversity because it is formed through the
interactions of a number of soil organisms: fungi, earthworms,
bacteria, plant roots, and burrowing mammals. Species loss
could result in soils unable to support vegetation.
Not all of the world’s supply of food comes from cultivated sources. There is still a substantial harvest of wild
plants and animals. Commercial fishing is in essence the
hunting of wild fish populations. Blueberries and maple syrup
are just two examples of foods gleaned from nature in the
United States. Wood and wood pulp are other products harvested from biodiversity resources. Biodiversity in itself is a
major economic force, as evidenced by the increasing popularity of ecotourism. Sport fishing, hunting, and bird-watching
are other examples of economically profitable activities that
depend on the preservation of biodiversity. Lastly, nearly half
of the medicinals now in use originated from a wild plant, and
it has been estimated that between 35,000 and 70,000 species of plants are used directly as medicines worldwide.
Knowledge of systematics has many practical applications.
There has been a movement to reduce our dependence
upon chemical pesticides and instead rely more heavily on
biological controls to manage nuisance organisms. Biological
control methods depend upon proper identification of a pest,
knowledge of its life cycle, and correct identification of its
predators and susceptibility to disease. Misidentification can
be costly. Mealybugs are noxious pests that can cause massive damage to crops. A species of mealybug was identified as
the culprit in the devastation of coffee plantations in Kenya.
Biological control methods were employed using the natural
enemies of the identified species of mealybug but were ineffective. Further investigation revealed that the mealybug had
been misidentified. Once the correct species was assigned,
natural pests of the mealybug were brought in from its native
habitat in Uganda, and the mealybug infestation was soon
brought under control. Knowledge of systematics can be
used to predict economic uses of little known but related
species. Researchers identified anticancer compounds from
Kenyan populations of Maytenus buchananii. There was a
problem, however; the species was rare in this locality.
Knowledge of systematics suggested that closely related species would probably possess the same chemical compounds.
This proved to be the case when a population of the same
genus but different species was collected from India.
Clearly, the preservation of biodiversity should be of
utmost importance to everyone. Systematic research is fundamental to learning about the characteristics and dimensions
of biodiversity. Systematics is necessary to identify localities
of high species diversity or rare species. Baseline data must
be collected to ascertain which species are declining in numbers or those whose range is becoming limited. Knowledge
of systematics will determine if exotic pests are moving into
new areas and threatening native species. Without scientific
identification and mapping, valuable habitats and the species
found there will be lost. In fact, Systematics Agenda 2000 is
an ongoing global initiative by the scientific community to discover, describe, and classify the world’s species in an effort
to understand and conserve biodiversity.
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Kingdom: Plantae
Plant kingdom
Figure 8.7 Major ranks in the taxonomic hierarchy. Note that
the higher the ranking, the broader the defining characteristics and
the more inclusive the group.
Table 8.5 Traditional and
Standardized Names for Some
Common Families
Division: Magnoliophyta
Class: Liliopsida
Flowering plants
Family
Name
Traditional
Name
Standardized
Name
Sunflower
Compositae
Asteraceae
Mustard
Cruciferae
Brassicaceae
Grass
Gramineae
Poaceae
Mint
Labiatae
Lamiaceae
Pea
Leguminosae
Fabaceae
Palm
Palmae
Arecaceae
Carrot
Umbelliferae
Apiaceae
Table 8.6 The Taxonomic
Hierarchy and Standard Endings
Monocots
Rank
Standard
Ending
Example
Division (Phylum)
-phyta
Magnoliophyta
Class
Order
Family
Genus
Order: Liliales
Species
Family: Liliaceae
Genus: Lilium
Lily family
Lilies
-opsida
Liliopsida
-ales
Liliales
-aceae
Liliaceae
Lilium
Lilium superbum L.
THE INFLUENCE OF DARWIN’S
THEORY OF EVOLUTION
The theory of evolution by means of natural selection was to
irrevocably change the way biologists view species. Instead
of unchanging organisms and generations created all alike, it
was realized that species are dynamic and variable, continually evolving through the mechanism of natural selection in
which adaptions are refined to a changing environment.
The Voyage of the HMS Beagle
Species: Lilium
superbum L.
Turk’s Cap Lily
Charles Robert Darwin (fig. 8.8) was born in England in
1809 to a family of distinguished naturalists and physicians. His grandfather was Erasmus Darwin, a well-known
poet and physician, and his father, Robert Darwin, was a
successful country doctor. At 15 years of age, Charles was
sent to the University of Edinburgh Medical School to study
medicine. Not finding it to his liking, he transferred after
Levetin−McMahon: Plants
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II. Introduction to Plant
Life: Botanical Principles
8. Plant Systematics and
Evolution
CHAPTER 8
Figure 8.8 Charles Darwin (1809–1882) published The Origin
of Species in 1859.
British
Isles
North
America
Galápagos
Islands
135
Plant Systematics and Evolution
2 years to Cambridge University to study theology. While at
Cambridge, he spent much of his free time with the students
and professors of natural history. This association later proved
invaluable. In 1831, at the age of 22, Darwin graduated from
Cambridge with a degree in theology. Shortly thereafter he
was recommended as ship naturalist by John Henslow, one of
the natural history professors at Cambridge. The ship in question was the HMS Beagle, commissioned by King William
IV to undertake a voyage around the world for the purpose of
charting coastlines, particularly that of South America, for the
British navy. The voyage of the Beagle began on December
27, 1831, and was to last 5 years (fig. 8.9). During his time
on the Beagle, Darwin collected thousands of plants and other
specimens from South America, the Galápagos Islands (off
the coast of Ecuador), Australia, and New Zealand. He studied geological formations and noted fossil forms of extinct
species. He found that some fossils of extinct species bore a
striking resemblance to extant species, as though the former
had given rise to the latter. Darwin spent some time studying
the species found on the Galápagos Islands. He noted that
animals and plants found in the Galápagos were obviously
similar to species found in South America, but there were
distinct differences. These observations led Darwin to question the fixity of species concept. According to this concept,
widely held at the time of Darwin, species were acts of Divine
Creation, unchanging over time.
When the Beagle returned to England in 1836, Darwin
married his cousin Emma Wedgwood (of the famous
Europe
Azores
Atlantic
Ocean
Asia
Canary
Islands
Cape Verde
Islands
Pacific
Ocean
Africa
Bahia
South
America
Rio de
Janeiro
Tahiti
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Valparaíso
Montevideo
Falkland
Islands
Cocos
Islands
Cape of
Good Hope
Australia
Sydney
King George
Sound
Tasmania
New
Zealand
Cape Horn
Figure 8.9 The 5-year voyage of the HMS Beagle. Darwin’s observations on the geology and distributions of plants and animals in
South America and the Galápagos Islands were the groundwork for the development of the theory of evolution by means of natural
selection.
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Wedgwood china family) and settled, at age 27, in the English
countryside. He continued his work in natural history, conducting experiments, writing papers, and corresponding with
other naturalists. Among his works was a four-volume treatise on the classification and natural history of barnacles.
In 1842, he began putting his thoughts together on what
was to become his theory of evolution by natural selection.
Darwin continued to expand and fine-tune his thoughts over
the next 16 years. In June of 1858, he received a manuscript
from Alfred Russel Wallace (1823–1913), a young British
naturalist working in Malaysia. Wallace’s work was entitled
On the Tendency of Varieties to Depart Indefinitely from
the Original Type; Wallace had independently arrived at
the concept of natural selection. Wallace and Darwin jointly
presented their ideas on July 1, 1858, at a meeting of the
Linnean Society in London. During the next few months,
Darwin completed writing what was to become one of the
most influential texts of all time. With the publication on
November 24, 1859 of On the Origin of Species by Means of
Natural Selection, or the Preservation of Favoured Races in
the Struggle for Life by Charles Darwin, biological thought
was changed forever.
Natural Selection
There are four underlying premises to Darwin’s theory of
evolution by natural selection:
1. Variation: Members within a species exhibit individual
differences, and these differences are heritable.
2. Overproduction: Natural populations increase geometrically, producing more offspring than will survive.
3. Competition: Individuals compete for limited resources,
what Darwin called “a struggle for existence.”
4. Survival to reproduce: Only those individuals that are
better suited to the environment survive and reproduce
(survival of the fittest), passing on to a proportion of their
offspring the advantageous characteristics.
Offspring that inherit the advantageous traits are selected
for survival and many will live to reproductive age passing on
the desirable attributes. Those that do not inherit these traits
are not likely to survive or reproduce. Gradually, the species
evolves, or changes, as more and more individuals carry these
traits. Darwin gave this example:
If the number of individuals of a species with plumed
seeds could be increased by greater powers of dissemination within its own area (that is, if the checks
to increase fell chiefly on the seeds), those seeds
which were provided with ever so little more down,
would in the long run be most disseminated; hence
a greater number of seeds thus formed would germinate, and would tend to produce plants inheriting
the slightly better-adapted down.
Concept Quiz
Darwin identified four conditions that are necessary if evolution is to occur: genetic variation, overproduction of offspring, competition for limited resources, and reproduction
of the fittest.
Imagine a plant population that reproduces entirely by asexual
methods, such as spreading by underground stems. Although
there are many individual plants in the population, they are
essentially a single plant genetically; that is, they are clones. Can
natural selection act on a population of clones? Is this population
capable of evolving? Explain.
In addition to natural selection, humans have long used
artificial selection, natural selection as practiced by humans
(see Chapter 11), to shape the characteristics of crop plants to
suit the needs of humanity. The most serious flaw in Darwin’s
Theory of Evolution was the mechanism of heredity. Darwin
had not worked out the source of variation in species, nor
did he understand the means by which traits are passed down
from generation to generation. It would take an Austrian
monk, Gregor Mendel (see Chapter 7), working in relative
obscurity with pea plants, to come up with the answers to
Darwin’s questions about inheritance.
A well-known example of natural selection is the case
of heavy-metal tolerance in bent grass, Agrostis tenuis.
Certain populations of bent grass were found growing near
the tailings, or soil heaps, excavated from lead mines in
Wales despite the fact that mine soils had high concentrations of lead and other heavy metals (copper, zinc, and
nickel). When mine plants were transplanted into uncontaminated pasture soil, all survived but were small and
slow growing. A nearby population of bent grass from
uncontaminated pasture soil exhibited no such tolerance
when transplanted into mine soil; in fact, most (57 out of
60) of the pasture plants died in the lead-contaminated
soil. The survival of the three pasture plants in mine soil is
significant; undoubtedly these three possessed an advantageous trait, the ability to tolerate heavy-metal soil. A trait
that promotes the survival and reproductive success of an
organism in a particular environment is an adaptation. The
mine plants had descended from bent grass plants that possessed the adaptation that conferred tolerance to the mine
soil; over time (less than 100 years in this case) populations
of Agrostis tolerant to heavy metal evolved from those few
tolerant individuals.
Although Darwin’s theory of natural selection is the
foundation of modern evolutionary concepts, biologists today
are still learning about the forces that shape evolution.
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CHAPTER 8
Concept Quiz
Natural selection favors the survivorship of those individuals in a population that possess characteristics crucial for
survival.
You observe that trees in a part of a forest in which deer are
plentiful have higher branches than the trees in a fenced-off part
of the forest. Explain the different selective forces at work in
these two different environments.
PHYLOCODE
The Linnaean system of nomenclature and the hierarchy of
classification that has been presented in this chapter were
created more than 250 years ago, before Charles Darwin and
Alfred R. Wallace had proposed their evolutionary theory
by means of natural selection. Linnaean nomenclature is an
artificial system based upon the appearance of organisms
that often does not reflect their evolutionary relationships, or
phylogeny. Currently, there is a movement to reject this preevolutionary taxonomy and replace it with a new system of
nomenclature, called PhyloCode, that is truly phylogenetic.
PhyloCode is based upon the work of the twentiethcentury German entomologist Willi Hennig, who proposed
that only shared derived characteristics should be used to
define a group of related organisms. He further proposed that
each group constructed should be monophyletic, or composed
of only those organisms that can trace their descent from a
common ancestor. These natural groupings are known as
clades.
First introduced in 1983, the PhyloCode abandons the
Linnaean ranks of the taxonomic hierarchy. In this system,
as new information that may change a group’s ranking accumulates, names are not changed, as they would be with the
Linnaean system, which associates different suffixes with
different ranks. Instead of ranks, clades are the only groups
recognized. Opponents fear that a complete abandonment of
all ranks will result in a loss of comparative information and
encourage a proliferation of names that, without any context,
will serve only to confuse the nomenclature.
Released in 1991, the APG (American Phylogeny Group)
system compared the sequence data of select genes to classify the flowering plants. The highest formal rank in this
classification system is the order; higher categories are only
identified as clades. As more data accumulated, APG II, an
update of the classification, became available in 2003. In this
system, angiosperms are recognized as a clade, sharing several
distinct characteristics, such as ovules enclosed in a carpel
and double fertilization. Within the angiosperms, all monocots appear to belong to a distinct clade, but molecular data
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137
indicates that the traditional dicots represent several evolutionary lineages. Most of the dicots do comprise a clade and are now
called the eudicots or true dicots. Approximately 75% of all
angiosperm species are now classified as eudicots. Traditional
dicots excluded from the clade eudicot are called the paleodicots (literally old dicots) by some authorities and include several ancient lineages in the evolution of angiosperms.
CHAPTER SUMMARY
1. Plant systematics has its origins in the classical works of
Theophrastus of ancient Greece, who is generally regarded
as the Father of Botany. The study of plants, as did many
other intellectual endeavors, went into a decline during
the Dark Ages of Europe but was later revived owing to
renewed interest in herbalism during the fifteenth to seventeenth centuries.
2. Linnaeus, a Swedish botanist of the eighteenth century,
is credited with the creation of the binomial, or scientific
name. Although common names are often informative and
readily accessible, scientific names have the advantage of
being recognized the world over and unique to a single
species.
3. The taxonomic hierarchy includes the major ranks:
domain, kingdom, division (phylum), class, order, family,
genus, and species.
4. Biologists have wrestled with the concept of the species;
the biological concept describes a species as a group of
interbreeding populations, reproductively isolated from
other populations.
5. Charles Darwin and his theory of evolution by natural
selection irrevocably changed the way biologists viewed
species. Natural selection favors those individuals that
possess traits that better enable them to survive in the
environment. These individuals survive to reproduce, and
many of their offspring will tend to have these adaptations
and pass them on to future generations. In this way, populations change over time. The four underlying conditions
of Darwin’s theory of evolution by natural selection are
variation, overproduction of offspring, competition, and
survival to reproduce.
REVIEW QUESTIONS
1. List the common names of some of the wildflowers in
your area. Determine the type of information each name
imparts.
2. Using a plant dictionary (see Further Reading) look up the
scientific names and their meanings for common houseplants and landscape plants in your area.
3. Briefly describe the concept of evolution by natural
selection.
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4. Why are only inherited traits important in the evolutionary process?
Laufer, Geraldine Adamich. 1996. Tussie-Mussies. The Herb
Companion April/May: 48–53.
5. How do mutations (Chapter 7) lead to the evolution of
new species?
6. What was the lasting contribution of Linnaeus? How was
the binomial system an improvement over polynomials?
7. In what ways can systematics preserve biodiversity?
Litt, Amy. 2006. Origins of Floral Diversity. Natural History
115(5): 34 –40.
FURTHER READING
Blunt, Wilfrid. 1971. The Compleat Naturalist: A Life of
Linnaeus. The Viking Press, New York, NY.
Briggs, David, and S. Max Walters. 1984. Plant Variation
and Evolution, 2nd Edition. Cambridge University Press,
Cambridge, MA.
Coombes, Allen J. 1994. Dictionary of Plant Names: Botanical
Names and Their Common Equivalents. Timber Press,
Beaverton, OR.
Conniff, Richard. 2006/2007. Happy Birthday Linnaeus.
Natural History 115(10): 42–47.
Darwin, Charles (author) and Edward O Wilson (editor) 2006.
From So Simple A Beginning: The Four Great Books of
Charles Darwin. W. W. Norton & Company, New York.
Friedman, William E. 2006. Sex Among the Flowers. Natural
History 115(9): 48–53.
Gilbert, Bil. 1984. The Obscure Fame of Carl Linnaeus.
Audubon 86:102–115.
Greenaway, Kate. 1992. Language of Flowers. Dover Press,
New York, NY.
Irvine, William. 1983. Apes, Angels, and Victorians: The
Story of Darwin, Huxley, and Evolution. University Press
of America, New York, NY.
Judd, Walter S., Christopher S. Campbell, Elizabeth A.
Kellogg, Peter F. Stevens, and Michael J. Donoghue.
2002. Plant Systematics, A Phylogenetic Approach, 2nd
Edition. Sinauer Associates, Sunderland, MA.
Kohn, David. 2005. The Miraculous Season: The Historical
Darwin. Natural History 114(9): 38–40.
Laufer, Geraldine Adamich. 1993. The Language of Flowers.
Workman Publishing, New York, NY.
Mayr, Ernst. 2000. Darwin’s Influence on Modern Thought.
Scientific American 283(1): 78–83.
Miller, Douglass R., and Amy Y. Rossman. 1995. Systematics,
Biodiversity, and Agriculture. BioScience 45(10): 680–
686.
Pennisi, Elizabeth. 2001. Linnaeus’s Last Stand? Science
291: 2304–2307.
Piementel, David, Christa Wilson, Christine McCullum,
Rachel Huang, Paulette Dwen, Jessica Flack, Quynh Tran,
Tamara Saltman, and Barabara Cluff. 1995. Economic
and Environmental Benefits of Biodiversity. BioScience
47(11): 747–757.
Quammen, David. 2004. Darwin’s Big Idea. National
Geographic 206(5): 2–35.
Quammen, David. 2007. The Name Giver. National
Geographic 211(6): 72–87.
Raby, Peter. 2001. Alfred Russel Wallace: A Life. Princeton
University Press, Princeton, NJ.
Savage, Jay M. 1995. Systematics and the Biodiversity Crisis.
BioScience 45(10): 673–679.
Schiebinger, Londa. 1996. The Loves of Plants. Scientific
American 274(2): 110–115.
Sulloway, Frank J. 2005. The Evolution of Charles Darwin.
Smithsonian 36(9): 58–69.
Withgott, Jay. 2000. Is It “So Long, Linnaeus?” BioScience
50(8): 646–651.
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