INTRODUCTION: WHAT'S IN A NAME?

"A rose is a rose," it has been said. And most of us know a rose when we see one, as we know the African marigolds we plant beside the potatoes and beans in our gardens, and the maples, elms, cedars, and pines that shade our backyards and line our streets. We usually call these plants by their common names. But if we wanted to know more about the cedar tree in our front yard, we would find that "cedar" may refer to an eastern red cedar, an incense cedar, a western red cedar, an Atlantic white cedar, a Spanish cedar, a banak cedar, or the biblical cedar of Lebanon. In fact, we would find that cedars are found in three separate plant families.

Later, after discovering that our "African" marigolds are in fact from Mexico and our "Spanish" cedar originated in the West Indies, we would realize how misleading the common names of plants can be. The same plant can have many different common names. The European white lily has at least 245, while the marsh marigold has at least 280. Clearly, if we use only the common name of a plant, we cannot be sure of understanding very much about that plant.

It is for this reason that the scientific community prefers to use a more precise way of naming, or classification. Scientific classification, however, is more than just naming: it is a key to understanding. Botanists name a plant to give it a unique place in the biological world, as well as to clarify its relationships within that world. Classification is sometimes difficult. As modern botany has advanced, producing an increasing understanding of complex biochemical mechanisms, the criteria for the way plants are classified has undergone transformation. Even further, Nature is not fixed and plants, like us, are capable of change. Plants can vary for reasons we don't entirely understand. To be sure, plant classification is not the dull field that some might assume!

HOW ARE PLANTS CLASSIFIED?

Science classifies living things in an orderly system through which they can be readily identified. Living things are grouped into categories of increasing size, based upon relationships within those categories. For example, all plants can be put in order from the more primitive to the more advanced. Such a ranking would look like this:

Plant Kingdom

Bryophytes: Small with leaflike, stemlike, and rootlike structures.

Disseminated by spores: mosses, liverworts, hornworts.

Vascular Plants: Larger with true leaves, stems, and roots.

1. Seedless: Ferns, horsetails, club mosses.

2. Seed Plants:
   1. Gymnosperms: Usually have cones, no flowers, seeds not enclosed in fruit: pines, spruces, firs, hemlocks, cycads, ginkgo.

   2. Angiosperms: Have flowers, seeds enclosed in fruit
      1. Monocotyledons: Leaves have parallel veins, one seed leaf: grasses, orchids, lilies, palms.

      2. Dicotyledons: Leaves have netted veins, two seed leaves: cherry trees, maples, coffee, daisies, etc.

This informal way of describing plant classification gives an overview of how plants are classified. Botanists use a more complex system. A botanist divides the plant kingdom into Divisions, similar to the Phyla used to divide the animal kingdom. There are twelve divisions. Referring to the above ranking, three of these divisions are Bryophytes, four are seedless plants, four are Gymnosperms, and one is Angiosperms. Each Division is further divided into Classes, which are divided into Orders, which are divided into Families, which are divided into Genera (singular, Genus), which are divided into species, which is the "basic unit" of classification. Put somewhat simply, individuals in a species are able to breed with each other, while in broader categories individuals do not interbreed.

THE BINOMIAL SYSTEM OF CLASSIFICATION

The foxglove plant is the source of digitalis. Photo by Steven R. King, 1996.

The scientific or botanical name of a plant is the means by which we give it its unique place in the scientific and biological world. Begun by Carolus Linneaus, a Swedish botanist, in the eighteenth century, this name is binomial (has two parts), consisting of genus and species, both of which are expressed in Latin. The genus or generic name is a noun which usually names some aspect of a plant, such as Coffea, the Latinized form of the Arabic word for beverage, kahwah. The species or specific name is usually an adjective that describes the genus. In the case of coffee, the species is arabica, indicating that the plant was thought to originate in Arabia. The coffee plant botanical name, Coffea arabica, refers to only one plant and cannot be confused with any other. Its botanical name is unique to that particular plant the world over.

The botanical name is often followed by a letter or letters which stand for the botanist who named that plant. The coffee plant's complete botanical name is Coffea arabica L., the L. standing for Linneaus. If the original botanical name of a plant is later changed, the original classifier is still noted in parentheses. Other often used abbreviations are Sarg. for Charles Sprague Sargent, founder of Harvard University's Arnold Arboretum; Lam. for Jean Baptiste Lamarck, French evolutionist and botanist; and Audub. for John James Audubon, ornithologist, naturalist, and painter. (Interestingly, this convention of naming the discoverer is not found in the naming of animals.) Sometimes the Family name is included, which groups the genera. It can usually be distinguished by its ending--"eae."

Linneaus's book Species Plantarum (The Species of Plants), published in 1753, continues to influence the naming of plants today. It is the starting point for checking whether a name has been used previously to insure that each plant is given a unique name. The earliest name for a plant is usually the official name should a dispute arise.

WHAT THE NAMES MEAN

Coca leaves and scale for sale in Bolivian market to local coca chewers. Photo by Steven R. King, 1996.

The genus and species names often tell something about the plant. They can describe the appearance of the plant, reflect the common name of the plant, indicate a chemical present in the plant, tell how the plant tastes or smells, or describe how the plant grows. The genus or species name can honor someone, a botanist, a person in power, someone historically prominent. The name can reflect the country or origin of a plant.

For example, Erythroxylum coca, the plant from which we derive cocaine, is named after erythro meaning red and xylo meaning wood, literally "red stem." (Coca, the species name, is the common name of the plant.) The jaborandi tree Pilocarpus jaborandi has a genus name which indicates that the alkaloid pilocarpine can be extracted from the plant. The species name jaborandi means "one who makes saliva or one who spits," referring to the use of the plant as an expectorant.

Plant classification can be painstakingly difficult. Plant species can resemble one another quite closely; plants can sometimes interbreed within species or across species, producing hybrids and varieties that complicate classification. A case in point is the cinchona tree, a plant instrumental in world history as a result of its alkaloid derivative, quinine, which helped to reduce the incidence of the terrible disease malaria. The cinchona tree, with its many species and hybrids and varieties within species, has resisted absolute classification. It's ambivalent ways have left botanists puzzled as to the exact number of species which exist. In fact, one species grouping of cinchona has been labeled 'Cinchona officinalis.' Officinalis (meaning 'of the workshop') is a common species name used for many medicinal plants, particularly, it seems, under the trying circumstances of difficult taxonomy.

Drawing of herbarium specimen of Cinchona officinalis, the source of quinine. Photo by Steven R. King, 1996.

PLANT CLASSIFICATION IN OUR MODERN WORLD

Despite the great advances made in botany, there are many, many plants yet to be discovered, classified, and utilized; unknown plants are treasures waiting to be found. Today's ethnobotanists are combing regions of the world, looking for tomorrow's medicines and food crops. They are exploring the functional properties and relationships of plants within ecosystems to help us to understand the need for diversity in the way we manage our plant resources.

The plant world, our world, is in constant flux. Due to human and other factors, we are seeing the possibility of extinction for many plants and animals. Plant classification aids in keeping track of our planet's endangered inhabitants. Just as importantly, we are realizing the need to understand ecological systems which preserve biodiversity. Today's scientists are exploring how genetic diversity and ecological sensitivity are necessary in solving such problems as feeding the population and fighting disease. Plant classification is vital to these endeavors. As is plain to see, a name is not just a name.

 

PLANT CLASSIFICATION

I. GENERAL TERMINOLOGY

        1. By growth habit:

Succulent plants - herbaceous or herbs (succulent seed plants possessing self-supporting stems)
Vine - a climbing or trailing herbaceous plant (Liana - a climbing or trailing woody plant)
Trees - having a single central axis
Shrub - having several more or less upright stems

2. By leaf drops:

Deciduous - no living leaves during dormant (winter) season (apple)
Evergreen - retaining functional leaves throughout the year (spruce)

3. By life span:

Annuals - plants that normally complete their life cycle during a single growing season (lettuce, spinach, marigold)
Biennial - plant that normally completes its life cycle during a period of two growing seasons (celery, carrot, parsnip)

Vegetative (often rosettes) during the first growing season. The winter following the first growing season provides the low temperature necessary to stimulate to 'bolt' or to send up a seed stalk during the second growing season. Carrots, radish and beets are harvested as annuals at the end of the first growing season after they develop over-wintering storage organs.

Perennial - plants that grow year after year, often taking many years to mature.

Unlike annuals and biennials, the perennial does not necessarily die after flowering (fruit trees; asparagus, rhubarb whose above ground parts are killed each year (in temperate regions) but roots remain alive to send up shoots in the spring; subtropical perennials such as tomato and eggplant are considered annual in temperate regions; Rubus (raspberries) has perennial roots and biennial shoots)

4. By temperature tolerance:

Tender plant - damaged or killed by low temperature
Hardy plant - withstands winter low temperatures
Wood hardy - a whole plant is winter hardy
Flower-bud hardiness - ability of flower buds to survive low winter temperatures (peach, ginkgo tree)

5. By temperature requirements:

Cool-season crop - prefers cool temperatures (peas, lettuce, cole crops)
Warm-season crop - prefers warm temperatures (tomato, pepper)

6. By habitat or site preference:

Xerophyte - prefers dry sites
Shade plants - prefers low light intensity
Acid loving - prefers low pH soils
Halophyte - prefers salty soils (in constrat to glycophyte)

II. HORTICULTURAL PLANT CLASSIFICATION

1. Edibles

A. Vegetables

Plants grown for aerial portions

Cole Crops(broccoli, cabbage, cauliflower)
Legumes (bean, peas)
Solanaceous fruit crops (Capsicum pepper, eggplant, tomato)
Cucurbits or vine crops (cucumber, melon, squash, pumpkin)
Greens or pot herbs (chard, dandelion, spinach)
Mushrooms (Agaricus, Lentinus)
Other vegetables(asparagus, okra, sweet corn)

Plants grown for underground portions

Root crops

Temperate (beet, carrot, radish, turnip)
Tropical (cassava, sweet potato, taro, yam)

Tuber crops (Jerusalem artichoke, potato)
Bulb and corm crops (garlic, onion shallot)

B. Fruits

Temperate (Deciduous)

Small fruits

Berries (blueberry, cranberry, strawberry)
Brambles (blackberry, raspberry)
Vines (grape, kiwifruit)

Tree fruits

Pome fruits (apple, pear, quince)
Stone fruits (apricot, cherry, peach, plum)

Subtropical and tropical (Evergreen)

Herbaceous and vine fruits (banana, papaya, passion fruit, pineapple)
Tree fruits

Citrus (grapefruit, lemon, lime, mandarin, orange)
Non-citrus (avocado, date, fig, mango, mangosteen)

C. Nuts

Temperate (almond, chestnut, filbert, pecan, pistachio)
Tropical (Brazil nut, cashew, macadamia)

D. Beverage Crops

Seed (cacao, coffea)
Leaf (tea, mate)

E. Herbs and Spices

Culinary herbs (dill, rosemary, sage)
Flavorings (peppermint, spearmint)
Tropical spices(cinnamon, clove, nutmeg, pepper)

2. Ornamentals

A. Florist Crops

Cut flowers (rose, carnation, chrysanthemum, alstroemeria)
Flowering pot plants (geranium, poinsettia, Easter lily, gloxinia)
Foliage plants (philodendron, ficus, aglaonema)
Bedding plants (petunia, impatiens, marigold, zinnia)

B. Landscape Plants

Trees

Deciduous (maple, elm, aspen, oak, willow)
Evergreen (pine, juniper, spruce)

Shrubs

Deciduous (lilac, azalea, privet)
Evergreen (juniper)

 Vines (ivy, bougainvillea, pyracantha)

Herbaceous perennials (penstemon, peony, columbine)

Ground covers (ivy, vinca, juniper)

C. Lawn and Turf Plants

Bermudagrass, bluegrass, fescue, perennial ryegrass, buffalograss

3. Industrial Crops

Drugs and Medicinals(digitalis, quinine, opium poppy)
Oil Seeds (oilpalm, jojoba, tung)
Extractives and Resins (Scotch pine, Para rubber tree)
Insecticides (pyrethrin, neam plant)

III. NOMENCLATURE (SCIENTIFIC PLANT CLASSIFICATION)

Early classification started by the Greek philosopher Theophrastus who classified all plants into annuals, biennials, and perennials according to life spans, and into herbs, shrubs, and trees according to their growth habits.

The modern taxonomy for plant classification is based on Linnaeus (a 18th century Swedish physician, now considered "father of taxonomy") who revolutionized the fields of plant and animal classification.

1. The Plant Kingdom

Kingdom Plantae
        Division Anthrophyta
                Class Dicotyledonae
                        Order Rosales
                                Family Rosaceae
                                        Genus Malus (or Malus)
                                                Species pumila (or pumila)
                                                        Variety
                                                                Form
                                                                    Individual

Horticulture deals with mostly family, genus, species, and cultivars.

- Botanical names are binomial.
- Underline or italicize genus and species:
        Genus species (or Genus species)
- Do not underline the family and cultivar names:

Rosaceae, Golden Delicious

- Variety names may be underlined.

 Examples:

Juniperus communis var. depressa (Prostrate Juniper)
Malus domestica cv. Red Delicious (Red Delicious Apple)
Malus domestica 'Red Delicious' (Red Delicious Apple)
Malus pumila cv. Red Delicious (Red Delicious Apple)

2. Some frequently used terms
 

Variety - a group of variants within a species which have similar characteristics.
Cultivar - cultivated variety
Ecospecies - a subdivision of species that are formed by ecological barrier. i.e., Cercis canadensis (Redbud)
Clone - a group of plants all of which arose from a single individual (the ortet) through asexual propagation.
Clonal cultivar - asexually propagated clones (potato, rose, etc.).
Pure line cultivar - homozygous inbred lines grown from seed.
Hybrid cultivar - a cultivar composed of hybrids between genetically diverse parental lines uniform phenotype, genetically heterozygous)

Differences Between Plants and Animals

Plants	Animals
Cellulose cell wall surrounds the cell membrane	Cell wall is absent in animal cells
Plastids are present especially a green pigment called chlorophyll	Chlorophyll is absent in animal cells
Large vacuoles containing cell sap are present in plant cell	Vacuoles are usually absent
Most plants do not exhibit movement of locomotion	Most animals exhibit movement of locomotion
Keep growing through out their life and are localized in the apical meristem	Growth stops after maturation but body cells are replaced periodically. Growth is uniform and proportionate
Manufacture their own food by photosynthesis	Cannot make their own food. They depend directly or indirectly on plants for their food
Sense organs and nervous system absent	Well - developed nervous system

 Let us study the classification of Plant Kingdom and Animal Kingdom in detail.

CLASSIFICATION

The Binomial System We are beginning a review of the diversity of plant life. First we need to look at how plants are classified.

	Both of these plants could be called bluebells but the one on the left is Scilla non-scripta and is related to lilies and hyacinths, whereas on the right is Mertensia virginica which is related to bugloss and lungwort.
Above Hypericum calycinum (family Hypericaceae) is called "Rose of Sharon" in England. Right Hibiscus syriacus (family Malvaceae) is called "Rose of Sharon" in North America
Plants fit into a system of classification common to all organisms, the binomial system, which was devised by Linnaeus. It is surprising how many plants and other organisms Linnaeus got to know. Many binomials are followed by the abbreviation L., indicating that Linnaeus described and named them. This is Linnaeus' favorite plant Linnaea borealis

The (Latin) binomial name is an unambiguous identifier. It consists of a genus name (with capital first letter) and a specific epithet (entirely in lower case) and is printed in italic or underlined. This is all it takes to identify an organism but we may put organisms together in larger groups for convenience.

Hierarchy of Classification

Kingdom	Plantae	Plantae
Division	Anthophyta	Anthophyta
Class	Dicotyledones	Monocotyledones
Family	Boraginaceae	Liliaceae
Genus	Mertensia	Scilla
Specific epithet	virginica	non- scripta
Common name	Virginia bluebell	English Bluebell

So a species is defined by the genus name and specific epithet along with the authority (such as L. for Linnaeus) who first gave a botanical description of the type specimen. Classification becomes complicated for cultivated plants since we often need to distinguish particular cultivars and many of these plants are hybrids of two or more wild species. If you see an X before the specific epithet such as Dendranthema X grandiflora for Chrysanthemum this tells you that the plant is a hybrid. It is more likely that species within a genus will be able to hybridize, but occasionally intergeneric hybrids are possible, such as X Cupressocyparis leylandii derived from a cross between Cupressus and Chamaecyparis species. Basis of Classification About 250,000 species of flowering plants are known. Species were first defined in terms of their appearance. And the first criterion for including a plant in a species should be that it looks similar to other members of the species. Plants are very variable in vegetative features, whereas flowers tend to be much less variable at least in basic structure. This is why flower features have been used so extensively in classification. We have come to use further criteria as we try to make our classification more "natural":

Members of a species should:	be able to interbreed and produce viable seed be physiologically similar occupy similar habitats

The Genus Further problems in classification arise as we move above the species level. A genus may include only one species, or a number that are similar enough to group together according to the same criteria that we use for species. Leaf shape and general appearance may be quite variable (although we do not often find a woody and a herbaceous plant in the same genus). The basic pattern of flower structure will stay the same even if there are variations in color, size and other details. Although intergeneric hybrids occur occasionally, hybrids between species in the same genus are much more common. Some people argue that if two species can be crossed to produce viable seed, then they belong in the same genus. The Family The most commonly used taxonomic grouping above the genus is the family. We will look at some of the 300 or so families within the flowering plants. Again there may be one or many genera in a family. In the Anthophyta, members of a family tend to share some important feature of flower structure as in the Asteraceae, or Araceae, although the variation can sometimes be bewildering as in the Rosaceae. Orders and classes Families can be grouped into orders such as the Geraniales which include the Geraniaceae (Geranium family), Linaceae (flax family) and Oxalidaceae (Sorrel family). However, orders are not much used as a taxonomic unit. On the other hand the two classes within the anthophyta, monocotyledones (monocots) and dicotyledones (dicots) are frequently mentioned. Divisions Above the family kingdoms are broken into divisions. In this course we will look at most of the divisions of the plant kingdom:

Division	Common name	Covered
Hepaticophyta	liverworts	yes
Anthoceropyta	hornworts	no
Bryophyta	mosses	yes
Psilophyta	whisk ferns	yes
Lycophyta	clubmosses	yes
Sphenophyta	horsetails	yes
Pterophyta	ferns	yes
Coniferophyta	conifers	yes
Cycadophyta	cycads	yes
Ginkgophyta	Maidenhair tree	yes
Gnetophyta	--	no
Anthophyta	flowering plants	yes

As we shall see, within each division there is something distinctive about the mode of reproduction and the morphology of the plants. Kingdoms In addition to plants we will look at selected Protista, Bacteria and Fungi that share some plant-like features and some of which are called plants by many people. We are using a traditional classification of organisms into five kingdoms and the one kingdom that we do not look at is Animalia. If you look through other textbooks you may see organisms move around. At one time multicellular algae were included in Plantae, whereas they are now grouped with unicellular organisms in the Protista. Also many fungus-like organisms have been moved into the Protista. Raven's "Biology of Plants" divides the bacteria into two kingdoms: Archaebacteria, an unusual group that may have been the ancestor of other kingdoms and Eubacteria which includes most of today's bacteria. This illustrates the fact that ideas about classification keep changing from the grand scale of kingdoms down to species and even sub-species. In recent years molecular characters have increasingly been used to decide taxonomic relationships and disputes. When we discuss "plant like" organisms it will be useful to have a list of plant features in mind for comparison. All members of the kingdom Plantae have: eukaryotic cells (containing vacuoles, chloroplasts and other organelles) cell walls composed of cellulose and pectin differentiated cell types photosynthesis, (utilizing chlorophylls a and b, and storing starch) adaptation for life on the land sexual reproduction through an alternation of generations Prokaryotes have various mechanisms for gene exchange but all eukaryotes have some method of sexual reproduction involving cellular fusion. This entails that at some time there must be a stage of segregation - otherwise we would go on increasing our chromosome number at every generation. Given that a fusion of haploid cells (gametes) leads to a diploid cell (zygote) which at some time undergoes meiosis to give rise to more haploid cells, there are two obvious ways that organisms could develop: (a) exist normally as haploid cells (single or multicellular), occasionally fuse to form a zygote which immediately undergoes meiosis to begin a new haploid generation or: (b) exist normally as diploid cells which occasionally undergo meiosis, followed more or less immediately by fusion to form a zygote which begins a new diploid generation There is however a third way which plants have adopted which combines both of these systems in an alternation of generations: (c) the organism can exist in both haploid and diploid states; the zygote grows into a diploid sporophyte in which some cells undergo meiosis to produce spores; the spores germinate and grow into a haploid gametophyte that produces male and/or female gametes; the gametes fuse to produce a new zygote Alternation of Generations Alternation of generations occurs throughout the plant kingdom right through to flowering plants and it is impossible to understand plant reproduction without grasping this concept. Some algae show this feature particularly well. For example in Ulva lactuca (Sea lettuce) the haploid and diploid generations are well developed and look identical until they produce gametes or spores, respectively: Life cycle of Ulva lactuca Exceptions to the rule

Sometimes confusion arises because a plant or group of plants lacks one of the features that we expect: it has no chlorophyll or is aquatic. If everything else about it is plant-like, we assume that it lost its other plant-like features in the course of evolution, so we still include it in the kingdom. This is Beech Drops (Epifagus virginiana) a common parasitic flowering plant that lives on the roots of beech trees in Ohio. It is brown rather than green because it lacks chlorophyll, but everything else about it is plant-like.

Plant Classification

Plant classification is an attempt to organize the hundreds of thousands of species of plants into a meaningful scheme that allows us to make assumptions about the life history, biology, and chemical constituents of various groups. The International Code of Botanical Nomenclature, the rules that govern the naming of plants, currently recognizes seven ranks. Each of these ranks is included in one (and only one) higher rank and has one or more members of subordinate rank.

The complete classification of corn, for example, with the names of each of the seven ranks of corn is:

Kingdom: Plantae
   Division: Magnoliophyta
      Class: Liliopsida
         Order: Cyperales
            Family: Poaceae
               Genus: Zea
                  Species: Zea mays

The way corn is classified at each of these seven ranks indicates something about the plant. The kingdom Plantae indicates that corn is a plant and probably photosynthesizes using chlorophyll. The division Magnoliophyta indicates that it is a flowering plant. The class Liliopsida indicates that corn is a monocot—that is, it has one cotyledon (the first leaf formed on a seedling) and leaves with parallel veins, and its flower parts occur in multiples of three (three or six petals, three or six stamens, and so on). The order Cyperales indicates that the seeds store starch and the flowers lack petals and sepals. The family Poaceae indicates that it has a unique type of inflorescence, round stems, and a distinctive internal anatomy. And so on, down to the species level.

The highest rank for living organisms is the kingdom. For many centuries, living organisms were grouped into one of two kingdoms—plant or animal. Only in the past several decades have new classifications arisen; with new discoveries and new understanding of life, this undoubtedly will continue to change. Today, the most widely accepted classification recognizes five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia.

Plant

Scientific classification
Domain:	Eukaryota
(unranked):	Archaeplastida
Kingdom:	Plantae Haeckel, 1866
Divisions
Green algae Chlorophyta Charophyta Land plants (embryophytes) Non-vascular land plants (bryophytes) Marchantiophyta—liverworts Anthocerotophyta—hornworts Bryophyta—mosses †Horneophytopsida Vascular plants (tracheophytes) †Rhyniophyta—rhyniophytes †Zosterophyllophyta—zosterophylls Lycopodiophyta—clubmosses †Trimerophytophyta—trimerophytes Pteridophyta—ferns and horsetails †Progymnospermophyta Seed plants (spermatophytes) †Pteridospermatophyta—seed ferns Pinophyta—conifers Cycadophyta—cycads Ginkgophyta—ginkgo Gnetophyta—gnetae Magnoliophyta—flowering plants †Nematophytes

Plants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The scientific study of plants, known as botany, has identified about 350,000 extant species of plants, defined as seed plants, bryophytes, ferns and fern allies. As of 2004, some 287,655 species had been identified, of which 258,650 are flowering and 18,000 bryophytes (see table below). Green plants, sometimes called Viridiplantae, obtain most of their energy from sunlight via a process called photosynthesis.

Definition

Aristotle divided all living things between plants (which generally do not move), and animals (which often are mobile to catch their food). In Linnaeus' system, these became the Kingdoms Vegetabilia (later Metaphyta or Plantae) and Animalia (also called Metazoa). Since then, it has become clear that the Plantae as originally defined included several unrelated groups, and the fungi and several groups of algae were removed to new kingdoms. However, these are still often considered plants in many contexts, both technical and popular.

Current definitions of Plantae

When the name Plantae or plants is applied to a specific taxon, it is usually referring to one of three concepts. From smallest to largest in inclusiveness, these three groupings are:

Name(s)	Scope	Description
Land plants, also known as Embryophyta or Metaphyta.	Plantae sensu strictissimo	As the narrowest of plant categories, this is further delineated below.
Green plants - also known as Viridiplantae, Viridiphyta or Chlorobionta	Plantae sensu stricto	Comprise the above Embryophytes, Charophyta (i.e., primitive stoneworts), and Chlorophyta (i.e., green algae such as sea lettuce). Viridiplantae encompasses a group of organisms that possess chlorophyll a and b, have plastids that are bound by only two membranes, are capable of storing starch, and have cellulose in their cell walls. It is this clade which is mainly the subject of this article.
Archaeplastida, Plastida or Primoplantae	Plantae sensu lato	Comprises the green plants above, as well as Rhodophyta (red algae) and Glaucophyta (simple glaucophyte algae). As the broadest plant clade, this comprises most of the eukaryotes that eons ago acquired their chloroplasts directly by engulfing cyanobacteria.

Outside of formal scientific contexts, the term "plant" implies an association with certain traits, such as multicellularity, cellulose, and photosynthesis.^[2][3] Many of the classification controversies involve organisms that are rarely encountered and are of minimal apparent economic significance, but are crucial in developing an understanding of the evolution of modern flora.

Algae

Most algae are no longer classified within the Kingdom Plantae.^[4][5] The algae comprise several different groups of organisms that produce energy through photosynthesis, each of which arose independently from separate non-photosynthetic ancestors. Most conspicuous among the algae are the seaweeds, multicellular algae that may roughly resemble terrestrial plants, but are classified among the green, red, and brown algae. Each of these algal groups also includes various microscopic and single-celled organisms.

The two groups of green algae are the closest relatives of land plants (embryophytes). The first of these groups is the Charophyta (desmids and stoneworts), from which the embryophytes developed.^[6][7][8] The sister group to the combined embryophytes and charophytes is the other group of green algae,Chlorophyta, and this more inclusive group is collectively referred to as the green plants or Viridiplantae. The Kingdom Plantae is often taken to mean this monophyletic grouping. With a few exceptions among the green algae, all such forms have cell walls containing cellulose, have chloroplasts containing chlorophylls a and b, and store food in the form of starch. They undergo closed mitosis without centrioles, and typically have mitochondria with flat cristae.

The chloroplasts of green plants are surrounded by two membranes, suggesting they originated directly from endosymbiotic cyanobacteria. The same is true of two additional groups of algae: the Rhodophyta (red algae) and Glaucophyta. All three groups together are generally believed to have a common origin, and so are classified together in the taxon Archaeplastida. In contrast, most other algae (e.g. heterokonts, haptophytes, dinoflagellates, and euglenids) have chloroplasts with three or four surrounding membranes. They are not close relatives of the green plants, presumably acquiring chloroplasts separately from ingested or symbiotic green and red algae.

Fungi

Fungi were previously included in the plant kingdom, but are now seen to be more closely related to animals. Unlike embryophytes and algae which are generally photosynthetic, fungi are often saprotrophs: obtaining food by breaking down and absorbing surrounding materials. Most fungi are formed by microscopic structures called hyphae, which may or may not be divided into cells but contain eukaryotic nuclei. Fruiting bodies, of which mushrooms are most familiar, are the reproductive structures of fungi. They are not related to any of the photosynthetic groups, but are close relatives of animals. Therefore, the fungi are in a kingdom of their own.

Diversity

About 350,000 species of plants, defined as seed plants, bryophytes, ferns and fern allies, are estimated to exist currently. As of 2004, some 287,655 species had been identified, of which 258,650 are flowering plants, 16,000 bryophytes, 11,000 ferns and 8,000 green algae.

Diversity of living plant divisions

Informal group	Division name	Common name	No. of living species
Green algae	Chlorophyta	green algae (chlorophytes)	3,800 [9]
	Charophyta	green algae (desmids & charophytes)	4,000 - 6,000 [10]
Bryophytes	Marchantiophyta	liverworts	6,000 - 8,000 [11]
	Anthocerotophyta	hornworts	100 - 200 [12]
	Bryophyta	mosses	12,000 [13]
Pteridophytes	Lycopodiophyta	club mosses	1,200 [5]
	Pteridophyta	ferns, whisk ferns & horsetails	11,000 [5]
Seed plants	Cycadophyta	cycads	160 [14]
	Ginkgophyta	ginkgo	1 [15]
	Pinophyta	conifers	630 [5]
	Gnetophyta	gnetophytes	70 [5]
	Magnoliophyta	flowering plants	258,650 [16]

The naming of plants is governed by the International Code of Botanical Nomenclature and International Code of Nomenclature for Cultivated Plants (see cultivated plant taxonomy).

Phylogeny

This article may be confusing or unclear to readers. Please help clarify the article; suggestions may be found on the talk page. (March 2009)

A proposed phylogeny of the Plantae after Kenrick and Crane^[17] is as follows, with modification to the Pteridophyta from Smith et al.^[18] The Prasinophyceae may be a paraphyletic basal group to all green plants.

Prasinophyceae (micromonads) Streptobionta Embryophytes Stomatophytes Polysporangiates Tracheophytes Eutracheophytes Euphyllophytina Lignophytia Spermatophytes (seed plants) Progymnospermophyta † Pteridophyta Pteridopsida (true ferns) Marattiopsida Equisetopsida (horsetails) Psilotopsida (whisk ferns & adders'-tongues) Cladoxylopsida † Lycophytina Lycopodiophyta Zosterophyllophyta † Rhyniophyta † Aglaophyton † Horneophytopsida † Bryophyta (mosses) Anthocerotophyta (hornworts) Marchantiophyta (liverworts) Charophyta Chlorophyta Trebouxiophyceae (Pleurastrophyceae) Chlorophyceae Ulvophyceae

Embryophytes

The plants that are likely most familiar to us are the multicellular land plants, called embryophytes. They include the vascular plants, plants with full systems of leaves, stems, and roots. They also include a few of their close relatives, often called bryophytes, of which mosses and liverworts are the most common.

All of these plants have eukaryotic cells with cell walls composed of cellulose, and most obtain their energy through photosynthesis, using light and carbon dioxideparasites on other species of photosynthetic plants. Plants are distinguished from green algae, which represent a mode of photosynthetic life similar to the kind modern plants are believed to have evolved from, by having specialized reproductive organs protected by non-reproductive tissues. to synthesize food. About three hundred plant species do not photosynthesize but are

Bryophytes first appeared during the early Paleozoic. They can only survive where moisture is available for significant periods, although some species are desiccation tolerant. Most species of bryophyte remain small throughout their life-cycle. This involves an alternation between two generations: a haploid stage, called the gametophyte, and a diploid stage, called the sporophyte. The sporophyte is short-lived and remains dependent on its parent gametophyte.

Vascular plants first appeared during the Silurian period, and by the Devonian had diversified and spread into many different land environments. They have a number of adaptations that allowed them to overcome the limitations of the bryophytes. These include a cuticle resistant to desiccation, and vascular tissues which transport water throughout the organism. In most the sporophyte acts as a separate individual, while the gametophyte remains small.

The first primitive seed plants, Pteridosperms (seed ferns) and Cordaites, both groups now extinct, appeared in the late Devonian and diversified through the Carboniferous, with further evolution through the Permian and Triassic periods. In these the gametophyte stage is completely reduced, and the sporophyte begins life inside an enclosure called a seed, which develops while on the parent plant, and with fertilisation by means of pollen grains. Whereas other vascular plants, such as ferns, reproduce by means of spores and so need moisture to develop, some seed plants can survive and reproduce in extremely arid conditions.

Early seed plants are referred to as gymnosperms (naked seeds), as the seed embryo is not enclosed in a protective structure at pollination, with the pollen landing directly on the embryo. Four surviving groups remain widespread now, particularly the conifers, which are dominant trees in several biomes. The angiosperms, comprising the flowering plants, were the last major group of plants to appear, emerging from within the gymnosperms during the Jurassic and diversifying rapidly during the Cretaceous. These differ in that the seed embryo (angiosperm) is enclosed, so the pollen has to grow a tube to penetrate the protective seed coat; they are the predominant group of flora in most biomes today.

Fossils

Plant fossils include roots, wood, leaves, seeds, fruit, pollen, spores, phytoliths, and amber (the fossilized resin produced by some plants). Fossil land plants are recorded in terrestrial, lacustrine, fluvial and nearshore marine sediments. Pollen, spores and algae (dinoflagellates and acritarchs) are used for dating sedimentary rock sequences. The remains of fossil plants are not as common as fossil animals, although plant fossils are locally abundant in many regions worldwide.

The earliest fossils clearly assignable to Kingdom Plantae are fossil green algae from the Cambrian. These fossils resemble calcified multicellular members of the Dasycladales. Earlier Precambrian fossils are known which resemble single-cell green algae, but definitive identity with that group of algae is uncertain.

The oldest known fossils of embryophytes date from the Ordovician, though such fossils are fragmentary. By the Silurian, fossils of whole plants are preserved, including the lycophyte Baragwanathia longifolia. From the Devonian, detailed fossils of rhyniophytes have been found. Early fossils of these ancient plants show the individual cells within the plant tissue. The Devonian period also saw the evolution of what many believe to be the first modern tree, Archaeopteris. This fern-like tree combined a woody trunk with the fronds of a fern, but produced no seeds.

The Coal measures are a major source of Paleozoic plant fossils, with many groups of plants in existence at this time. The spoil heaps of coal mines are the best places to collect; coal itself is the remains of fossilised plants, though structural detail of the plant fossils is rarely visible in coal. In the Fossil Forest at Victoria Park in Glasgow, Scotland, the stumps of Lepidodendron trees are found in their original growth positions.

The fossilized remains of conifer and angiosperm roots, stems and branches may be locally abundant in lake and inshore sedimentary rocks from the Mesozoic and Cenozoic eras. Sequoia and its allies, magnolia, oak, and palms are often found.

Petrified wood is common in some parts of the world, and is most frequently found in arid or desert areas where it is more readily exposed by erosion. Petrified wood is often heavily silicified (the organic material replaced by silicon dioxide), and the impregnated tissue is often preserved in fine detail. Such specimens may be cut and polished using lapidary equipment. Fossil forests of petrified wood have been found in all continents.

Fossils of seed ferns such as Glossopteris are widely distributed throughout several continents of the Southern Hemisphere, a fact that gave support to Alfred Wegener's early ideas regarding Continental drift theory.

Structure, growth, and development

Most of the solid material in a plant is taken from the atmosphere. Through a process known as photosynthesis, most plants use the energy in sunlight to convert carbon dioxide from the atmosphere, plus water, into simple sugars. Parasitic plants, on the other hand, use the resources of its host to grow. These sugars are then used as building blocks and form the main structural component of the plant. Chlorophyll, a green-colored, magnesium-containing pigment is essential to this process; it is generally present in plant leaves, and often in other plant parts as well.

Plants usually rely on soil primarily for support and water (in quantitative terms), but also obtain compounds of nitrogen, phosphorus, and other crucial elemental nutrients. Epiphytic and lithophytic plants often depend on rainwater or other sources for nutrients and carnivorous plants supplement their nutrient requirements with insect prey that they capture. For the majority of plants to grow successfully they also require oxygen in the atmosphere and around their roots for respiration. However, some plants grow as submerged aquatics, using oxygen dissolved in the surrounding water, and a few specialized vascular plants, such as mangroves, can grow with their roots in anoxic conditions.

Factors affecting growth

The genotype of a plant affects its growth, for example selected varieties of wheat grow rapidly, maturing within 110 days, whereas others, in the same environmental conditions, grow more slowly and mature within 155 days.^[19]

Growth is also determined by environmental factors, such as temperature, available water, available light, and available nutrients in the soil. Any change in the availability of these external conditions will be reflected in the plants growth.

Biotic factors are also capable of affecting plant growth. Plants compete with other plants for space, water, light and nutrients. Plants can be so crowded that no single individual produces normal growth. Optimal plant growth can be hampered by grazing animals, suboptimal soil composition, lack of mycorrhizal fungi, and attacks by insects or plant diseases, including those caused by bacteria, fungi, viruses, and nematodes.^[19]

Simple plants like algae may have short life spans as individuals, but their populations are commonly seasonal. Other plants may be organized according to their seasonal growth pattern: annual plants live and reproduce within one growing season, biennial plants live for two growing seasons and usually reproduce in second year, and perennial plants live for many growing seasons and continue to reproduce once they are mature. These designations often depend on climate and other environmental factors; plants that are annual in alpine or temperate regions can be biennial or perennial in warmer climates. Among the vascular plants, perennials include both evergreens that keep their leaves the entire year, and deciduous plants which lose their leaves for some part of it. In temperate and boreal climates, they generally lose their leaves during the winter; many tropical plants lose their leaves during the dry season.

The growth rate of plants is extremely variable. Some mosses grow less than 0.001 millimeters per hour (mm/h), while most trees grow 0.025-0.250 mm/h. Some climbing species, such as kudzu, which do not need to produce thick supportive tissue, may grow up to 12.5 mm/h.

Plants protect themselves from frost and dehydration stress with antifreeze proteins, heat-shock proteins and sugars (sucrose is common). LEA (Late Embryogenesisdesiccation and freezing.^[20] Abundant) protein expression is induced by stresses and protects other proteins from aggregation as a result of

Plant cell

Plant cells are typically distinguished by their large water-filled central vacuole, chloroplasts, and rigid cell walls that are made up of cellulose, hemicellulose, and pectin. Cell division is also characterized by the development of a phragmoplast for the construction of a cell plate in the late stages of cytokinesis. Just as in animals, plant cells differentiate and develop into multiple cell types. Totipotent meristematic cells can differentiate into vascular, storage, protective (e.g. epidermal layer), or reproductive tissues, with more primitive plants lacking some tissue types.^[21]

Physiology

Photosynthesis

Plants are photosynthetic, which means that they manufacture their own food molecules using energy obtained from light. The primary mechanism plants have for capturing light energy is the pigment chlorophyll. All green plants contain two forms of chlorophyll, chlorophyll a and chlorophyll b. The latter of these pigments is not found in red or brown algae.

Internal distribution

Vascular plants differ from other plants in that they transport nutrients between different parts through specialized structures, called xylem and phloem. They also have roots for taking up water and minerals. The xylem moves water and minerals from the root to the rest of the plant, and the phloem provides the roots with sugars and other nutrient produced by the leaves.^[21]

Ecology

The photosynthesis conducted by land plants and algae is the ultimate source of energy and organic material in nearly all ecosystems. Photosynthesis radically changed the composition of the early Earth's atmosphere, which as a result is now 21% oxygen. Animals and most other organisms are aerobic, relying on oxygen; those that do not are confined to relatively rare anaerobic environments. Plants are the primary producers in most terrestrial ecosystems and form the basis of the food web in those ecosystems. Many animals rely on plants for shelter as well as oxygen and food.

Land plants are key components of the water cycle and several other biogeochemical cycles. Some plants have coevolved with nitrogen fixing bacteria, making plants an important part of the nitrogen cycle. Plant roots play an essential role in soil development and prevention of soil erosion.

Distribution

Plants are distributed worldwide in varying numbers. While they inhabit a multitude of biomes and ecoregions, few can be found beyond the tundras at the northernmost regions of continental shelves. At the southern extremes, plants have adapted tenaciously to the prevailing conditions. (See Antarctic flora.)

Plants are often the dominant physical and structural component of habitats where they occur. Many of the Earth's biomes are named for the type of vegetation because plants are the dominant organisms in those biomes, such as grasslands and forests.

Ecological relationships

Numerous animals have coevolved with plants. Many animals pollinate flowers in exchange for food in the form of pollen or nectar. Many animals disperse seeds, often by eating fruit and passing the seeds in their feces. Myrmecophytes are plants that have coevolved with ants. The plant provides a home, and sometimes food, for the ants. In exchange, the ants defend the plant from herbivores and sometimes competing plants. Ant wastes provide organic fertilizer.

The majority of plant species have various kinds of fungi associated with their root systems in a kind of mutualistic symbiosis known as mycorrhiza. The fungi help the plants gain water and mineral nutrients from the soil, while the plant gives the fungi carbohydrates manufactured in photosynthesis. Some plants serve as homes for endophytic fungi that protect the plant from herbivores by producing toxins. The fungal endophyte, Neotyphodium coenophialum, in tall fescue (Festuca arundinacea) does tremendous economic damage to the cattle industry in the U.S.

Various forms of parasitism are also fairly common among plants, from the semi-parasitic mistletoe that merely takes some nutrients from its host, but still has photosynthetic leaves, to the fully parasitic broomrape and toothwort that acquire all their nutrients through connections to the roots of other plants, and so have no chlorophyll. Some plants, known as myco-heterotrophs, parasitize mycorrhizal fungi, and hence act as epiparasites on other plants.

Many plants are epiphytes, meaning they grow on other plants, usually trees, without parasitizing them. Epiphytes may indirectly harm their host plant by intercepting mineral nutrients and light that the host would otherwise receive. The weight of large numbers of epiphytes may break tree limbs. Hemiepiphytes like the strangler fig begin as epiphytes but eventually set their own roots and overpower and kill their host. Many orchids, bromeliads, ferns and mosses often grow as epiphytes. Bromeliad epiphytes accumulate water in leaf axils to form phytotelmata, complex aquatic food webs.^[22]

Approximately 630 plants are carnivorous, such as the Venus Flytrap (Dionaea muscipula) and sundew (Drosera species). They trap small animals and digest them to obtain mineral nutrients, especially nitrogen and phosphorus.^[23]

Importance

The study of plant uses by people is termed economic botany or ethnobotany; some consider economic botany to focus on modern cultivated plants, while ethnobotany focuses on indigenous plants cultivated and used by native peoples. Human cultivation of plants is part of agriculture, which is the basis of human civilization. Plant agriculture is subdivided into agronomy, horticulture and forestry.

Food

Much of human nutrition depends on land plants, either directly or indirectly.
Human nutrition depends to a large extent on cereals, especially maize (or corn), wheat and rice. Other staple crops include potato, cassava, and legumes. Human food also includes vegetables, spices, and certain fruits, nuts, herbs, and edible flowers.
Beverages produced from plants include coffee, tea, wine, beer and alcohol.
Sugar is obtained mainly from sugar cane and sugar beet.
Cooking oils and margarine come from maize, soybean, rapeseed, safflower, sunflower, olive and others.
Food additives include gum arabic, guar gum, locust bean gum, starch and pectin.
Livestock animals including cows, pigs, sheep, and goats are all herbivores; and feed primarily or entirely on cereal plants, particularly grasses.

Nonfood products

Wood is used for buildings, furniture, paper, cardboard, musical instruments and sports equipment. Cloth is often made from cotton, flax or synthetic fibers derived from cellulose, such as rayon and acetate. Renewable fuels from plants include firewood, peat and many other biofuels. Coal and petroleum are fossil fuels derived from plants. Medicines derived from plants include aspirin, taxol, morphine, quinine, reserpine, colchicine, digitalis and vincristine. There are hundreds of herbal supplements such as ginkgo, Echinacea, feverfew, and Saint John's wort. Pesticides derived from plants include nicotine, rotenone, strychnine and pyrethrins. Drugs obtained from plants include opium, cocaine and marijuana. Poisons from plants include ricin, hemlock and curare. Plants are the source of many natural products such as fibers, essential oils, dyes, pigments, waxes, tannins, latex, gums, resins, alkaloids, amber and cork. Products derived from plants include soaps, paints, shampoos, perfumes, cosmetics, turpentine, rubber, varnish, lubricants, linoleum, plastics, inks, chewing gum and hemp rope. Plants are also a primary source of basic chemicals for the industrial synthesis of a vast array of organic chemicals. These chemicals are used in a vast variety of studies and experiments.

Aesthetic uses

Thousands of plant species are cultivated for aesthetic purposes as well as to provide shade, modify temperatures, reduce wind, abate noise, provide privacy, and prevent soil erosion. People use cut flowers, dried flowers and houseplants indoors or in greenhouses. In outdoor gardens, lawn grasses, shade trees, ornamental trees, shrubs, vines, herbaceous perennials and bedding plants are used. Images of plants are often used in art, architecture, humor, language, and photography and on textiles, money, stamps, flags and coats of arms. Living plant art forms include topiary, bonsai, ikebana and espalier. Ornamental plants have sometimes changed the course of history, as in tulipomania. Plants are the basis of a multi-billion dollar per year tourism industry which includes travel to arboretums, botanical gardens, historic gardens, national parks, tulip festivals, rainforests, forests with colorful autumn leaves and the National Cherry Blossom Festival. Venus Flytrap, sensitive plant and resurrection plant are examples of plants sold as novelties.

Scientific and cultural uses

Tree rings are an important method of dating in archeology and serve as a record of past climates. Basic biological research has often been done with plants, such as the pea plants used to derive Gregor Mendel's laws of genetics. Space stations or space colonies may one day rely on plants for life support. Plants are used as national and state emblems, including state trees and state flowers. Ancient trees are revered and many are famous. Numerous world records are held by plants. Plants are often used as memorials, gifts and to mark special occasions such as births, deaths, weddings and holidays. Plants figure prominently in mythology, religion and literature. The field of ethnobotany studies plant use by indigenous cultures which helps to conserve endangered species as well as discover new medicinal plants. Gardening is the most popular leisure activity in the U.S. Working with plants or horticulture therapy is beneficial for rehabilitating people with disabilities. Certain plants contain psychotropic chemicals which are extracted and ingested, including tobacco, cannabis (marijuana), and opium.

Negative effects

Weeds are plants that grow where people do not want them. People have spread plants beyond their native ranges and some of these introduced plants become invasive, damaging existing ecosystems by displacing native species. Invasive plants cause billions of dollars in crop losses annually by displacing crop plants, they increase the cost of production and the use of chemical means to control them affects the environment.

Plants may cause harm to people and animals. Plants that produce windblown pollen invoke allergic reactions in people who suffer from hay fever. A wide variety of plants are poisonous to people and/or animals. Several plants cause skin irritations when touched, such as poison ivy. Certain plants contain psychotropicchemicals, which are extracted and ingested or smoked, including tobacco, cannabis (marijuana), cocaine and opium. Smoking causes damage to health or even death, while some drugs may also be harmful or fatal to people. Both illegal and legal drugs derived from plants may have negative effects on the economy, affecting worker productivity and law enforcement costs. Some plants cause allergic reactions in people and animals when ingested, while other plants cause food intolerances that negatively affect health.

Classification of Plants

Plants are classified in several different ways, and the further away from the garden we get, the more the name indicates a plant's relationship to other plants, and tells us about its place in the plant world rather than in the garden. Usually, only the Family, Genus and species are of concern to the gardener, but we sometimes include subspecies, variety or cultivar to identify a particular plant.

Starting from the top, the highest category, plants are classified as follows. Each group has the characteristics of the level above it, but has some distinguishing features. The further down the scale you go, the more minor the differences become, until you end up with a classification which applies to only one plant.

CLASS	Angiospermae (Angiosperms)	Plants which produce flowers
	Gymnospermae (Gymnosperms)	Plants which don't produce flowers
SUBCLASS	Dicotyledonae (Dicotyledons, Dicots)	Plants with two seed leaves
	Monocotyledonae (Monocotyledons, Monocots)	Plants with one seed leaf
SUPERORDER	A group of related Plant Families, classified in the order in which they are thought to have developed their differences from a common ancestor. There are six Superorders in the Dicotyledonae (Magnoliidae, Hamamelidae, Caryophyllidae, Dilleniidae, Rosidae, Asteridae), and four Superorders in the Monocotyledonae (Alismatidae, Commelinidae, Arecidae, Liliidae) The names of the Superorders end in -idae
ORDER	Each Superorder is further divided into several Orders. The names of the Orders end in -ales
FAMILY	Each Order is divided into Families. These are plants with many botanical features in common, and is the highest classification normally used. At this level, the similarity between plants is often easily recognisable by the layman. Modern botanical classification assigns a type plant to each Family, which has the particular characteristics which separate this group of plants from others, and names the Family after this plant. The number of Plant Families varies according to the botanist whose classification you follow. Some botanists recognise only 150 or so families, preferring to classify other similar plants as sub-families, while others recognise nearly 500 plant families. A widely-accepted system is that devised by Cronquist in 1968, which is only slightly revised today. Links to the various methods of classification are on this website. The names of the Families end in -aceae
SUBFAMILY	The Family may be further divided into a number of sub-families, which group together plants within the Family that have some significant botanical differences. The names of the Subfamilies end in -oideae
TRIBE	A further division of plants within a Family, based on smaller botanical differences, but still usually comprising many different plants. The names of the Tribes end in -eae
SUBTRIBE	A further division, based on even smaller botanical differences, often only recognisable to botanists. The names of the Subtribes end in -inae
GENUS	This is the part of the plant name that is most familiar, the normal name that you give a plant - Papaver (Poppy), Aquilegia (Columbine), and so on. The plants in a Genus are often easily recognisable as belonging to the same group. The name of the Genus should be written with a capital letter.
SPECIES	This is the level that defines an individual plant. Often, the name will describe some aspect of the plant - the colour of the flowers, size or shape of the leaves, or it may be named after the place where it was found. Together, the Genus and species name refer to only one plant, and they are used to identify that particular plant. Sometimes, the species is further divided into sub-species that contain plants not quite so distinct that they are classified as Varieties. The name of the species should be written after the Genus name, in small letters, with no capital letter.
VARIETY	A Variety is a plant that is only slightly different from the species plant, but the differences are not so insignificant as the differences in a form. The Latin is varietas, which is usually abbreviated to var. The name follows the Genus and species name, with var. before the individual variety name.
FORM	A form is a plant within a species that has minor botanical differences, such as the colour of flower or shape of the leaves. The name follows the Genus and species name, with form (or f.) before the individual variety name.
CULTIVAR	A Cultivar is a cultivated variety, a particular plant that has arisen either naturally or through deliberate hybridisation, and can be reproduced (vegetatively or by seed) to produce more of the same plant. The name follows the Genus and species name. It is written in the language of the person who described it, and should not be translated. It is either written in single quotation marks or has cv. written in front of the name.

Example of Classification

The full botanical classification of a particular Lesser Spearwort with narrow leaves is

Category	Scientific Name	Common Name
CLASS	Angiospermae	Angiosperms
SUBCLASS	Dicotyledonae	Dicotyledons
SUPERORDER	Magnoliidae	Magnolia Superorder
ORDER	Ranunculares	Buttercup Order
FAMILY	Ranunculaceae	Buttercup Family
SUBFAMILY	Ranunculoideae	Buttercup Subfamily
TRIBE	Ranunculeae	Buttercup Tribe
GENUS	Ranunculus	Buttercup
SPECIES	(Ranunculus) flammula	Lesser Spearwort
SUBSPECIES	(Ranunculus flammula) subsp. flammula	Lesser Spearwort
VARIETY	(Ranunculus flammula subsp. flammula) var. tenuifolius	Narrow-leaved Lesser Spearwort