In biology, a plant usually refers to a living organism in Kingdom Plantae. In daily use, it may not be the case.
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 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 are parasitic.
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: ...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.
Classification of the Plantae Kingdom
Simple nonvascular plants
Complex nonvascular plants
Bryophyta - mosses
Hepatophyta - liverworts
Anthocerophyta - hornworts
Psilophyta - whisk ferns
Lycophyta - club mosses
Sphenophyta - horsetails
Pterophyta - ferns
Pinophyta - conifers
Cycadophyta - cycads
Ginkgophyta - ginkgo
Gnetophyta - Gnetae
Magnoliophyta - flowering plants
All organisms of the Kingdom Plantae find their origins among a group called the green algae, which are paraphyletic to the remaining forms, and are variously included here or among the Protista. Green algae have chloroplasts containing chlorophylls a and b, bound by double membranes, and 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:
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 year.
Perennial: live for many growing seasons; continue to reproduce once mature.
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.
Plants may also be organized according to how they are used. Food plants include fruits, vegetables, herbs, and spices.