Horticulture Principles and Practices

Some superficial differences automatically place organisms into distinct classes.
For example, there are plants and there are animals. In plants, some bear flowers, others
do not; some have broad leaves, others have narrow leaves; some bear fleshy fruits, others
bear grains; and so on. These natural systems of classification are arbitrary and reflect
the uses human cultures have for plants. Other forms of nomenclature are based on
scientific principles that have universal application. This chapter explores the origin and
nature of these different systems for grouping and naming plants.
2.1 SCIENTIFIC AND BOTANICAL SYSTEMS
OF CLASSIFICATION
Taxonomy
The science of identifying,
naming, and classifying
plants.
Binomial
Nomenclature
A system of naming plants
whereby a plant is given a
two-part name
representing the genus
and species.
Kingdom
The highest taxonomic
category.
Species
A category of biological
classification ranking
immediately below the
genus or subgenus.
Order
A category of taxonomic
classification ranking
above the family and
below the class.
Family
A group of related plants
or animals forming a
category ranking above a
genus and below an order
and usually comprising
several to many genera.
Scientific systems of classification go beyond the superficial or natural system by
employing a number of criteria that include morphological, anatomical, ultrastructural,
physiological, phytochemical, cytological, and evolutionary (phylogenetical) criteria.
Pyrame de Candole is credited with the introduction of the term taxonomy as the science
of classifying and naming plants. Taxonomy is sometimes used synonymously with
systematics. The latter, however, is a field of biology involved with the study of diversity
among organisms to establish their natural (evolutionary) relationships, making taxonomy
a discipline of systematics.
In plant identification, individuals are assigned to a descending series of related
plants, based on their known common characteristics. For example, a marigold plant is
first placed in a more distant group with plants that have seed, then among seed plants
with flowers, and eventually in the most closely associated groups of varieties of
marigold. In terms of botanical nomenclature, Carolus Linnaeus is credited with developing
the current Latin-based system called the binomial nomenclature, because an individual
is given two names, as opposed to the polynomial system, which was more
descriptive. The international body that sets the rules for naming plants by this system
publishes the International Code of Botanical Nomenclature (ICBN) to provide guidelines
for standardizing the naming of plants. These rules are revised as new scientific
evidence becomes available.
2.2 TAXONOMIC GROUPS
Several general classification categories have been defined in organisms. These classifications
can be arranged in order from the most inclusive group (kingdom) to the least
inclusive group (species) (Figure 2–1). Each of these groups constitutes a taxon (plural:
taxa). In addition to these basic groups, subcategories are used in certain cases. These
include levels such as subdivision, subclass, suborder, subspecies, and variety
(or cultivar). An example of plant classification is presented in Table 2–1.
TABLE 2–1
An Example of Scientific Classification of Plants
Taxon Example Common Name
Kingdom Plantae Plant
Division Magnoliophyta Flowering plant
Class Liliopsida Monocot
Order Liliales Lily order
Family Liliaceae Lily family
Genus
Allium
Species Allium cepa Onion
30 Chapter 2 Classifying and Naming Horticultural Plants