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2 Classification of the Plantae Kingdom 3 Plant Categories 4 See also |
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". 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.
Difficulties in the Definition
Classification of the Plantae Kingdom
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Simple nonvascular plants
Green algae Complex nonvascular plants Bryophyta - mosses Hepatophyta - liverworts Anthocerophyta - hornworts Vascular plants Seedless Psilophyta - whisk ferns Lycophyta - club mosses Sphenophyta - horsetails Ophioglossophyta Pterophyta - ferns Seeded (Spermatophyta) 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:
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.
Plant Categories
Plants may also be organized according to how they are used. Food plants include fruits, vegetables, herbs, and spices.
See also
simple:Plant