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 organss or voluntary motion and has, when complete, a root, stem, and leavess. However, botanically only vascular plants have "a root, stem, and leaves", and even some vascular plants, such as some carnivorous plants and duckweed, fall afoul of the definition. 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 photoautotrophic at all, but saprophytic. And there are more than a few species of flowering plants, fungi, and bacteria that are parasitic.
Contemporary biological classification systems (see cladistics) tend to emphasize genetic relationships between organisms as the basis of classification. Ideally, a taxon (or clade) should be monophyletic; all of the organisms in the taxon or clade should share a single common ancestor, and the taxon or clade should include all descendants of that common ancestor. Another way to define the Plant Kingdom would be to determine whether all of the organisms in the kingdom can be traced to a common ancestor.
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. 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.
