Plant
From PlantWiki
The terms Plant and Kingdom Plantae are imprecisely defined. In everyday use, by lay people, these terms are used principally in reference to the embryophytes (bryophytes and vascular plants). The Plant concept, however, has evolved greatly in parallel with scientists' understanding of evolution and phylogeny; thus, many alternative definitions exist. There is no scientific consensus as to how these terms should be used.
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History
Two kingdom system
All organisms were classified by Linnaeus as being either plants or animals (the two kingdom system). Plants included Archaea, Bacteria, Fungi, algae, bryophytes, vascular plants, and slime moulds.
Three kingdom system
When it was recognized that Prokaryotes were a fundamentally distinct group, the original concept of "plant" and "Kingdom Plantae" was discarded. In the new three kingdom system, the Bacteria and Archaea were transferred to the new Kingdom Monera. This considerably narrowed the concept of plants.
Five kingdom system
(Plantae=Embryophyta)
It was eventually recognised that the three kingdom system was also flawed. A five kingdom system was introduced, with a new Kingdom Fungi, and a Kingdom Protista (or Proctoctista). In this system, the concept of "plants" and "Kingdom Plantae" was further narrowed, and limited to include only bryophytes and vascular plants. All eukaryotic algae were transferred to the Kingdom Protista. The Protista however was always an ill-defined assemblage of organisms - consisting of diverse organisms not fitting conveniently within the other kingdoms.
"Clade" System
(Plantae=Archaeplastida)
The flaws in the five kingdom system became especially apparent with the advent of modern, detailed molecular phylogenetic studies. Baldauf recognised that the eukaryotes consisted of a series of different clades (e.g., Opisthokonts, Heterokonts, etc). These clades had little correspondence with the older five kingdom system. This is forcing a major reconsideration of the kingdom system. One of the new clades was labelled "Plants". This clade comprised the Glaucophyta, Rhodophyta, Chlorophyta, other green algae, bryophytes and vascular plants.
The "Plant" clade has now been renamed Archaeplastida. The major subdivisions of the Archaeplastida are the Glaucophyta, Rhodophyta, and Viridiplantae. The Viridiplantae comprise the Chlorophyta, other green algae, bryophytes and vascular plants. The term embryophyte can be used to collectively and unambiguously refer to the bryophyte and vascular plant collective.
The "Green Plant Kingdom"
(Plantae=Viridiplantae)
Although there is no scientific consensus, the terms plant and Kingdom Plantae have sometimes recently been used as synonyms for Viridiplantae. This definition comprises the Chlorophyta, other green algae, bryophytes and vascular plants. It comprises all plants and algae which have the following characteristics: chlorophyll a and b, plastids bound by only two membranes, store starch, cellulose cell walls.