What is a plant?

Such a simple question, "What is a plant?" But the answer isn't so simple, and the Phactor very much does not many of the answers found when he googled this question. The ecological perspective appears to prevail, to wit, a plant is a photosynthetic producer. This means that cyanobacter, green sulfur bacteria, green non-sulfur bacteria, heliobacteria, and purple bacteria are by this definition plants. OK rather unsatisfying for a number of reasons, but primarily because these are bacteria. At least one definition tried to skirt this problem by inserting the modifier "eukaryotic" (nucleated cells) into the ecological definition. However, among eukaryotic photosynthetic producers are euglenoids, the brown algae, diatoms, and all manner of related phytoplanktonic organisms, and even the quite cool looking chlorarachniophytes, phytoplankton that looks like little green spiders. Except way down at the base of the eukaryotes none of these organisms are related to the green photosynthetic organisms you were first thinking of when you thought of plants, in fact they occupy at least three distinct lineages of organisms. Even if plants are restricted to a single lineage there is disagreement if this constitutes a plant kingdom, members of which are correctly called plants. No one disputes that land plants are plants, and some attempts have been made to narrowly them as the plant kingdom. But these plants have a common ancestry with a group of algal organisms called the Charophytes, and the land plants and charophytes have a common ancestry with the rest of the green algae. Although somewhat more in question, the green algae and land plant lineage is hypothesized to have a common ancestry with red algae, and maybe even some phytoplankton called glaucophytes. The entire lineage, one of five major eukaryote lineages, could be called a plant kingdom, or if you dislike using the old taxonomic categories, the plant clade. The take home message is that plants are one of many photosynthetic producers, and there are even a few non-photosynthetic producers among prokaryotes, so plant does not equal producer even in an ecological context. A plant is a member of the plant kingdom if only we could all agree on what that constitutes.