Encyclopedia  |   World Factbook  |   World Flags  |   Reference Tables  |   List of Lists     
   Academic Disciplines  |   Historical Timeline  |   Themed Timelines  |   Biographies  |   How-Tos     
Sponsor by The Tattoo Collection
Main Page | See live article | Alphabetical index


This article is about plants as living organisms. For other uses, see Plant (disambiguation)

In biology, the name plant is usually given to living organisms in Kingdom Plantae. In daily use, the term may include other additional organisms, as described below.

Table of contents
1 Difficulties in the definition
2 Evolution and classification of the Plant Kingdom
3 Plant categories
4 Plants as fossils
5 General plant information
6 References and further reading
7 See also
8 External links

Difficulties in the definition

The term plant is far more difficult to define than might be obvious. Although botanists describe a Kingdom Plantae, the boundaries defining members of Plantae are more exclusive than common definitions of "plant". We are tempted to regard plant as meaning a multicellular, eukaryotic organism that generally does not have sensory organss or voluntary motion and has, when complete, a root, stem, and leavess. However, botanically only vascular plants have a root, stem, and leaves, and even some vascular plants, such as some carnivorous plants and duckweed, fall afoul of the definition. But to be fair, the vascular plants are the plants we tend to encounter every day.

Another, much broader definition for plant is that it refers to anything that is photoautotrophic — that is, produces its own food from raw inorganic materials and sunlight. This is not an unreasonable definition, and one that focuses on the role plants typically play in an ecosystem. However, there are photoautotrophs among the Prokaryotes, specifically photoautotrophic bacteria and cyanophytes. The latter are sometimes called (for good reasons) blue-green algae. Then there arises the problem that most people, including botanists, would call a mushroom a plant, although a mushroom is the fruiting body of a fungus (Kingdom Fungi), and not photoautotrophic at all, but saprophytic. And there are more than a few species of flowering plants, fungi, and bacteria that are parasitic.

Contemporary biological classification systems (see cladistics) tend to emphasize genetic relationships between organisms as the basis of classification. Ideally, a taxon (or clade) should be monophyletic; all of the organisms in the taxon or clade should share a single common ancestor, and the taxon or clade should include all descendants of that common ancestor. Another way to define the Plant Kingdom would be to determine whether all of the organisms in the kingdom can be traced to a common ancestor.

We cannot offer a firm answer. The list of characteristics that separate the Plantae from the other biological kingdoms provides at least a technical definition. The problem this lack of precision or agreement in the definition of "plant" presents is one of understanding statements, often encountered in Wikipedia articles, of the sort: ...xylem is one of the two transport tissues of plants. In general it cannot be assumed this means all plants, algae through flowering plants. It very probably does not include fungi or bacteria. Indeed, it is usually safest to assume the discussion is about vascular plants (essentially the ferns, conifers, flowering plants, and a few others) unless stated differently (e.g., ...in vascular and non-vascular plants this is such and such).

The system of classification (see Scientific classification) employed by biologists to catalogue the earth's living organisms is one to which thousands of scientists daily devote a tremendous number of man-hours. The system devised attempts to be a "natural" one, defining the evolutionary relationships between all the different species (including those known only from fossils). Plants are a part of that categorization effort and whether defining "plant" narrowly or broadly, we must include some reference to the classification system in any scholarly effort to gain or give information about them.

Evolution and classification of the Plant Kingdom

Scientific classification
Domain: Eukaryota
Kingdom: Plantae
Green algae
land plants (embryophytes)
non-vascular embryophytes
    Hepatophyta - liverworts
    Anthocerophyta - hornworts
    Bryophyta - mosses
vascular plants (tracheophytes)
seedless vascular plants
   Lycopodiophyta - club mosses
   Equisetophyta - horsetails
   Pteridophyta (Filicopsida) ferns
      Psilotophyta whisk ferns
      Leptosporangiate or "true" ferns
seed plants (spermatophytes)
   Pinophyta - conifers
   Cycadophyta - cycads
   Ginkgophyta - ginkgo
   Gnetophyta - gnetae
   Magnoliophyta - flowering plants
Kingdom Plantae (or Viridaeplantae) is a
monophyletic group of eukaryotes. Over 60 major lineages of eukaryotes have been identified, most of which are unicellular and classified in the paraphyletic kingdom Protista. Kingdom Plantae comprises those eukaryotes that photosynthesize using chlorophylls a and b, store their photosynthetic products as starch inside the chloroplasts in which they are produced, have chloroplasts that are bounded by a double membrane, and have cell walls made of cellulose. The Kingdom includes several groups of green algae that evolved from the common ancestor of green plants. Green algae come in a variety of forms: flagellate, colonial, filamentous, and even primitively multicellular. Many are primarily haploid, but others exhibit alternation of generations between haploid and diploid forms, called the gametophyte and sporophyte.

Some time during the Palaeozoic, complex, multicellular plants (the Embryophytes) began to appear on land. In these early new forms, the gametophyte and sporophyte become very different in shape and function, the sporophyte remaining small and dependent on its parent for its whole brief life. Groups at this level of organization, collectively called bryophytes, include:

All of these forms are small and confined to moist environments, relying on water to disperse spores. In the Silurian, new embryophytes appeared with adaptations enabling them to overcome these constraints, which underwent a massive adaptive radiation in the Devonian period, taking over the land. These groups typically have a cuticle resistant to desiccation, and vascular tissue, which transports water throughout the organism, and are called vascular plants as a result. In many of these the sporophyte acts as a separate individual, with the gametophyte remaining very small. Taxa of vascular plants include:

The spermatophytes, or seed plants are a group of vascular plants which diversified towards the end of the Palaeozoic. In these forms it is the gametophyte that is completely reduced, and the young sporophyte begins life inside an enclosure called a seed, which develops on its parent. The living groups of spermatophytes include:

These divisions are grouped into gymnosperms (naked seeds; first four), and the flowering plants or angiosperms. The angiosperms are the last major group of plants to appear, emerging during the Jurassic, with a major radiation in the Cretaceous that resulted in their becoming the predominant land plants in most biomes.

Plant categories

In addition to the scientific classification of plants, or our more populist approach based upon that system, we may want to classify plants in a variety of other ways, some of which are considered here.

Plants may be organized according to their seasonal growth pattern. Of course simple plants like algae have individually short life spans and the following terms do not apply, but algae populations are commonly seasonal.

Vascular plants are either herbaceous (nonwoody) or woody. Woody plants may be trees with one or several trunks and branching occurring well above ground, or shrubs with no significant trunk, and branching occurring near ground surface.

Perennial vascular plants may be either evergreen, and keep their leaves all year round, or deciduous and lose their leaves for part of the year. Many deciduous plants, usually found in temperate and boreal climates, lose their leaves in the winter months, and some tropical and subtropical plants lose their leaves during the dry season.

Plants may also be organized according to how they are used. Food plants include fruits, vegetables, herbs, and spices.

Plants as fossils

Plant fossils include roots, wood, leaves, seeds, fruit, pollen and amber (the fossilized resin produced by some plants). Fossil plants are relatively less important than animal fossils in geology, although pollen and algae are used for dating some rocks. The remains of fossil plants are not as common as fossil animals, although plant fossils are locally abundant in many regions worldwide.

Early fossil plants are well known from the Devonian period chert of Rhynie in Aberdeenshire, Scotland. The best preserved examples, from which their cellular construction has been described, have been found at this locality. The preservation is so perfect that sections of these ancient plants show the individual cells within the plant tissue.

The Coal Measures are a major source of Palaeozoic plant fossils, with many groups of advanced plants in existance at this time. The spoil heaps of coal mines are the best places to collect. 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 angiosperm roots, stems and branches may be locally abundant in lake and inshore sedimentary rocks from the Mesozoic and Caenozoic eras. Oak Quercus and palm Palmoxylon are the most commonly found.

Petrified wood is very common in many parts of the world, and is most frequently found in arid or desert areas. Petrified wood is often heavily silicified, and the impregnated tissue is often preserved in fine detail. Such specimens may be cut and polished using special lapidary equipment. Fossil forests of petrified wood are found throughout Europe, North America and Africa.

General plant information

The growth rate of plants is extremely variable, from below 1 µm/h (e.g. some mosses), typically 25-250 µm/h in most trees, and up to 12,500 µm/h in some climbing species which do not need to divert materials into producing thickening supportive tissue (e.g. Pueraria montana)

References and further reading

See also

External links