Taxonomic Groups

Seven general classification categories have been defined in plants. These classifications can be arranged in order from the most inclusive group (kingdom)to the least inclusive group (species). Each of these groups constitutes a taxon (plural: taxa).In addition to these basic groups, subcategories are used in certain cases. These include such as subdivision, subclass, suborder, subspecies, and variety (or cultivar). An example of plant classification is presented in Table 2.1.

According to the binomial nomenclature, each individual has two-part name; the first part is called the genus (plural: genera)and second part is called a specific epithetor species. This system is equivalent to surnames and first names in the naming of people.

Kingdoms

Placing organisms into groups is a work in progress. Traditionally, living organisms are recognized as belonging to one of two categories or kingdom: the plant kingdomor the animalkingdom.However, as science advances and knowledge increases, this scheme periodically comes under review. From the two-kingdom scheme evolved the three-, four-, and five-kingdom classifications of organisms. Even the five-kingdom scheme is deemed inadequate by R. H. Whittaker (1969) proposed the five-kingdom classification. The criteria for the classification are cellular structure (complexity) and forms of nutrition (photosynthesis, ingestion, or absorption of food in solution).

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 order
Genus	Allium
Species	Allium cepa	Onion

Even though horticulture focuses on organisms in Kingdom Plantae, other kingdoms are directly or indirectly important to the field. Horticulture exists because of humans (Kingdom Animalia). The field was developed by humans. Organisms in the Kingdom Monera, Protoctista, and Fungi include those that are pathogen of horticultural plants, namely, bacteria, fungi, and viruses.

1. Kingdom Monera: Monera is a kingdom of unicellular (one-called) organisms (2.11). They are called prokaryotes and have no nuclear membrane or compartmentalization into distinct organelles. They are produce primarily by cell division and are mostly heterotrophic (cannot  make organic compounds and thus feed on material made by others). Bacteria are classified under this kingdom.
2. Kingdom production: The Kingdom Production includes algae (green, brown, and red), slime molds, and eukaryotes (cells with a nuclear membrane and compartmentalization).
3. Kingdom Fungi: Fungi are filamentous eukaryotes that lack plastids and the photosynthetic pigment (chlorophyll). Thus, they feed on dead or living organisms. Most plant diseases are caused by fungi.
4. Kingdom Animalia:   Kingdom Animalia consists of multicellular organisms that are eukaryotes but without cell walls, plastids, and capacity for photosynthesis (processing of food production from the sun by plants). Animals generally ingest their food and reproduce primarily by sexual means. Animals have the highest level of organization and tissue differentiation of any organism in any kingdom. They have complex sensory and neuromotor systems.  
5. Kingdom Plantae: Organisms in the Kingdom Plantae are photosynthetic (make food from inorganic materials; few plants are heterotrophic, that is, feeding on organic material from other sources). They are multicellular, have cell walls, and live on land.

Table 2.2. The Five Kingdoms of Organisms as Described by Whittaker
1. Monera (Have Prokaryotic Cells)
    Bacteria
2. Protoctista (Have Eukaryotic Cells)
    Algae
    Slime molds
    Flagellate fungi
    Protozoa
    Sponges
3. Fungi (Absorb Food in Solution)
    True fungi
4. Plantae (Produce Own Food by the Process of Photosynthesis)
    Bryophytes
    Vascular plants
5. Animalia (Ingest Their Food)
    Multicellular animals

Divisions of Kingdom Plantae
Several divisions are recognized in the Kingdom Plantae. These divisions can be divided into two major categories: bryophytes (nonvascular plants-the mosses, hornworts, and liverworts) and vascular plants. Vascular plants are large bodied and have three primary vegetative organs, stem, leaves, and roots, and also conducting tissues (vascular tissue). Vascular plants may produce seeds or be seedless. Most plants of horticultural interest are vascular plants.
In terms of relative abundance, more than 80 per cent of all species in the plant kingdom are flowering plants. Even though gymnosperms (seed plants whose seeds are not enclosed within an ovary during development) make up only 0.2 per cent of species in the plant kingdom, conifers (e.g., pines) occur on about one-third of forested lands of the world.
Variety versus Cultivar
The lowest and least-inclusive taxon is the species, as already indicated. Species may be subdivided into specific categories. A botanical variety is a naturally occurring variant of the species that is significantly different from the general species originally described. Botanical varieties may differ in subtle or more visible ways, such as in color, shape, size, chemical quality, or some other traits. Instead of two names, as expected in the binomial nomenclature, a variety requires the use of a third name after the introduction of the abbreviation var. (for variety). For example, broccoli is called Brassica oleraceae var. botrytis.
Through plant breeding, humans sometimes create new variants that are maintained under human supervision (as opposed to being naturally maintained, as is the case in varieties). The product of plant breeding is called a cultivar, a. contraction of two terms-cultivated and variety.

Table 2.3. The Divisions of the Kingdom Plantae

		Divisions	Common Name
Bryophytes	(Seedless)	Hepaticophyta	Liverworts
		Anthocerotophyta	Hornworts
		Bryophyta	Mosses
Vascular plants	(Seeded)	Psilotophyta	Whisk ferns
		Lycophyta	Club mosses
		Sphenophyta	Horsetails
		Pterophyta	Ferns
		Pinophyta	Gynosperms
		Subdivision: Cycadicae	Cycads
		Subdivision: Pinicae
		Class: Ginkgoatae	Ginkgo
		Class: Pinatae	Conifers
		Subdivision: Gneticae	Gnetum
		Magnoliophyta	Flowering plants
		Class: Liliopsida	Monocots
		Class: Mangoliopsida	Dicots

Cultivars are maintained as clones in vegetatively propagated (increasing the number of plants by using plant parts other than seed) species and as lines in species propagated by seed under specific conditions. Many flowers and vegetables have cultivars that are propagated by seed, whereas others are hybrids (F1 seed from a cross of two different parents)

Rules in Classification
In plant taxonomy, the ending of a name is often characteristic of the taxon. Classes often end in - opsida (e.g., Magnoliopsida). Names ending in -ae are subclasses of class names (depending on the classification system). Exceptions include several families such as Compositae (now called Asteraceae). Plant orders end in -ales (e.g., Rosales [roses]) while family names end in -aceae (e.g., Rosaceae).

These higher-order taxa are not routinely encountered, unless one is conducting taxonomic studies. The binomial names (genus and species) are the most frequently encountered. When you walk through a botanical garden or even a college campus where there is good horticulture program or a good grounds and gardens department, you may find that some plants in the landscape are labeled with the correct binomial name or scientific name, as well as the common name. The family name is quite frequently indicated.
For writing names following rules have been framed at the binomial level:

1. The entire binary name must be underlined or written in italics.
2. The genus name starts with an uppercase letter, and the species name is written in lowercase throughout. The term species is both singular and plural. It may be shortened to spp., for the
   plural species.
3. In technical writing, an initial L. may follow the species, indicating that Linnaeus first named the plant. Other abbreviations may be encountered in the literature. An example of a full binary name for corn, for example, Zea mays L. The genus may be abbreviated (e.g., Z mays L.). Some plants may have a subspecies and hence a third name added to the binary name. In such a case, the third name is also underlined or italicized.
4. Whereas the generic name can be written alone to refer to individuals in the group, the specific epithet cannot be used by itself (i.e., Zea but not mays).
5. At the bottom of the taxa hierarchy is variety, which is the naturally occurring and very closely related variant. As previously indicated, the binomial name is followed by the abbreviation var. and then the variety name. Cultivar names are not underlined or italicized  (e.g., Lycopersicon esculentum Mill. Cultivar 'Big Red' or L. esculentum cv. 'Big Red').
Specific epithets are adjectival in nature. Many genera can have the same specific epithet. Some of them indicate color, such as alba (white), vahegata (variegated), rub rum (red), and aureum (golden). Example are, vulgaris (common), esculentus (edible), sativus (cultivated), tuberosum (bearing tubers), and officinalis (medicinal).

In developing new horticultural cultivars, plant breeders employ a variety of techniques. The conventional techniques involve crossing or hybridizing plant that differ in desirable characteristics. In terms of taxonomic hierarchy, hybridization can be routinely performed at the base of the hierarchy (i.e., among varieties or cultivars of the same species). Crossing at other levels such as among species (interspecies hybridization leads to genetic complications. Such a cross is problematic and has limited success, requiring the use of additional technique, such as embryo rescue, in some cases.