Defining 'Plant'

Like many words in common usage that apply to biological entities or concepts, the term plant is more difficult to define than might be at first obvious. Although botanists describe a Kingdom Plantae, the boundaries defining members of Plantae are more exclusive than our common concept of a "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, and even some vascular plants, such as certain carnivorous plants and duckweed, fall afoul of that definition. But to be fair, the vascular plants are the plants we tend to encounter every day and that most people would readily regard as "plants".

A more significant point of departure between Plantae and plants occurs among the seaweeds. Technically, only a relatively minor group of seaweeds (the chlorophytes or green algae) are members of the Kingdom Plantae. The majority of seaweeds are not Plantae; but are they not plants? Certainly if we regard the green algae as plants, it is difficult to exclude the more prominent red and brown algae of our coastal waters.

Another, much broader definition for plant is that it refers to any organism that is photoautotrophic—produces its own food from raw inorganic materials and sunlight. This is not an unreasonable definition, and is 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 many people would consider that a mushroom is a plant; a mushroom is the fruiting body of a fungus (Kingdom Fungi) and not photoautotrophic at all, but saprophytic. However, there are more than a few species of flowering plants, fungi, and bacteria that are not autotrophic, but parasitic.

We cannot hope to offer a firm answer. The list of characteristics that separate the Plantae from the other biological kingdoms provides at least a technical definition, but realize it is only a technical definition. The problem this lack of precision or agreement in the definition of "plant" presents is one of understanding statements, often encountered in Wikipedia (and other) 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; see discusion below on "General Terminology") unless stated differently (e.g., ...in vascular and non-vascular plants this is such and such).

From Study Guide to the Science of Botany on Wikibooks.org. This text and its source is copyright under the GNU Free Documentation License.