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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 organs or voluntary motion and has,
when complete, a root, stem, and leaves. However, botanically only vascular
plants have "a root, stem, and leaves". But to be fair, the vascular plants
are the plants we tend to encounter every day.
Another, much broader (more inclusive) 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 photoautotophic at all, but saprophytic. And there
are more than a few species of flowering plants, fungi, and bacteria that
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, but not one likely to ever be popularly employed. That is, the
term "plant" will always mean more than the organisms classified in the
Plant Kingdom. For example, if "green" algae are clearly plants being
members of the Plantae, then most folks are not likely to exclude the
majority of seaweeds that are also algae (Kingdom Protista), but not green
algae. 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: Blank is a structure in plants that does
such and such. In general it cannot be assumed this means all plants, algae
through flowering plants. It very probably does not include fungi or
bacteria. It is 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
* For plants as defined in the Kingdom Plantae see below.
* For algae in general go here or see Seaweed.
* For mushrooms go here or see Fungi.
* For nonvascular plants in general go here.
* For vascular plants in general go here.
* For coniferous plants see Pinophyta or gymnosperms.
* For flowering plants see angiosperms.
Classification of the Plant Kingdom
All organisms of the Kingdom Plantae Scientific classification
find their origins among a group
called the green algae, which are Domain: Eukaryota
paraphyletic to the remaining forms, Kingdom: Plantae
and are variously included here or
among the Protista. Green algae have Divisions
chloroplasts containing chlorophylls Simple nonvascular plants
a and b, bound by double membranes, ΚΚΚ Green algae
and come in a variety of forms: Complex nonvascular plants
flagellate, colonial, filamentous, ΚΚΚ Bryophyta - mosses
and even primitively multicellular. ΚΚΚ Hepatophyta - liverworts
Many are primarily haploid, but ΚΚΚ Anthocerophyta - hornworts
others exhibit alternation of Vascular plants
generations between haploid and Seedless
diploid forms, called the gametophyte ΚΚΚ Psilophyta - whisk ferns
and sporophyte ΚΚΚ Lycophyta - club mosses
ΚΚΚ Sphenophyta - horsetails
Some time during the Palaeozoic ΚΚΚ Ophioglossophyta
plants began to appear on land. In ΚΚΚ Pterophyta - ferns
these new forms, the gametophyte and Seeded
sporophyte become very different in ΚΚΚ Pinophyta - conifers
shape and function, the sporophyte ΚΚΚ Cycadophyta - cycads
remaining small and dependent on its ΚΚΚ Ginkgophyta - ginkgo
parent for its whole brief life. ΚΚΚ Gnetophyta - Gnetae
Groups at this level of organization, ΚΚΚ Magnoliophyta - flowering plants
collectively called bryophytes,
* Division Bryophyta (mosses)
* Division Anthocerotophyta (hornworts)
* Division Hepaticophyta (liverworts)
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. Groups at
this level of organization include:
* Division Lycophyta (club mosses)
* Division Sphenophyta (horsetails)
* Division Psilophyta (whisk ferns)
* Division Ophioglossophyta (adders-tongues and grape-ferns)
* Division Pterophyta (ferns)
The vascular plants also include as a subgroup the spermatophytes, or seed
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.
* Division Cycadophyta (Cycads)
* Division Ginkgophyta (Ginkgo)
* Division Pinophyta (Conifers, Coniferophyta)
* Division Gnetophyta (Gnetae)
* Division Magnoliophyta (Flowering plants, Anthophyta)
These divisions are grouped into gymnosperms (naked seeds; first four), and
the flowering plants or angiosperms. The latter are the last major group of
plants to have appeared, arising during the Jurassic and quickly becoming
predominant in most biomes.
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.
* Annual: live and reproduce within one growing season.
* Biennial: live for two growing seasons; usually reproduce in second
* Perennial: live for many growing seasons; continue to reproduce once
Vascular plants are either herbaceous (nonwoody) or woody. Woody plants may
be trees with one or several trunks and branching occuring well above
ground, or shrubs with no significant trunk, and branching occuring near
Plants may also be organized according to how they are used. Food plants
include fruits, vegetables, herbs, and spices.