Plant

From Wikipedia, the free encyclopedia

Jump to: navigation, search

For other uses, see Plant (disambiguation).

Plants Fossil range: Cambrian to recent, but see text
Scientific classification
Domain: Eukaryota (unranked) Archaeplastida Kingdom: Plantae Haeckel, 1866[1]
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 a major group of life forms and include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. 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 and 15,000 bryophytes (see table below). Green plants, sometimes called metaphytes, obtain most of their energy from sunlight via a process called photosynthesis.

Contents [hide] 1 Definition 1.1 Algae 1.2 Fungi 2 Diversity 2.1 Phylogeny 2.2 Embryophytes 2.3 Fossils 3 Life processes 3.1 Growth 3.1.1 Factors affecting growth 3.2 Internal distribution 4 Ecology 4.1 Distribution 4.2 Ecological relationships 5 Importance 5.1 Food 5.2 Nonfood products 5.3 Aesthetic uses 5.4 Scientific and cultural uses 5.5 Negative effects 6 See also 7 References 7.1 Further reading 8 External links 8.1 Botanical and vegetation databases

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. Indeed, an attempt to perfectly match "plant" with a single taxon is problematic, because for most people the term "plant" is only vaguely related to the phylogenic concepts on which modern taxonomy and systematics are based.

When the name Plantae 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:

• Land plants, also known as Embryophyta or Metaphyta. As the narrowest of plant categories, this is further delineated below.
• Green plants -- also known as Viridiplantae, Viridiphyta or Chlorobionta -- comprise the above Embryophytes, Charophyta (i.e., primitive stoneworts), and Chlorophyta (i.e., green algae such as sea lettuce). It is this clade which is mainly the subject of this article.
• Primoplantae -- also known as Plantae sensu lato, Plastida, or Archaeplastida -- comprises the green plants above, 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.

Informally, other creatures that carry out photosynthesis are called plants as well, but they do not constitute a formal taxon and represent species that are not closely related to true plants. There are around 375,000 species of plants, and each year more are found and described by science.

Algae

Green algae from Ernst Haeckel's Kunstformen der Natur, 1904.

Main article: Algae

The algae comprise several different groups of organisms that produce energy through photosynthesis. However, most are not classified within the Kingdom Plantae but in the Kingdom Protista. Most conspicuous are the seaweeds, multicellular algae that may roughly resemble terrestrial plants, but are classified among the green, red, and brown algae. These and other algal groups also include various single-celled organisms.

The embryophytes developed from green algae (Chlorophyta); the two groups are 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 and 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 the red algae, and the two groups are generally believed to have a common origin (see Archaeplastida). In contrast, most other algae have chloroplasts with three or four membranes. They are not close relatives of the green plants, presumably in origin acquiring chloroplasts separately from ingested or symbiotic green and red algae.