Plant

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A plant is an organism of the kingdom Plantae, within the empire of Telluria. Plants are generally sessile and photosynthetic, and frequently compose the primary flora of terrestrial worlds, including Earth. More than a third of a million species of plant exist on Earth alone; other worlds include millions more, some of which are ambulatory or otherwise remarkably different from Euterran varieties. (Because of the dominance of plants in Euterran flora, laymen often erroneously use "plant" as a synonym for "florum".)

The study of plants is known as botany. A specialist in botany is called a botanist.

Contents

Classification

Along with the red algae and the obscure glaucophytes, plants are (somewhat tentatively) classified in the superkingdom of Archaeplastida, or Primoplantae. In turn, the kingdom Plantae comprises a number of divisions, the largest of which (at least on True Earth) is Magnoliophyta, the flowering plants. Magnoliophyta belongs to the superdivision Spermatophyta, the seed plants, along with the much smaller divisions Pinophyta (the conifers), Cycadophyta (cycads), Gnetophyta (an obscure group of plants called the gnetophytes), and Ginkgophyta (encompassing today only the single extant species of the ginkgo, Ginkgo biloba, though other species and genera existed in prehistoric times). Other divisions of plants include Pteridophyta, comprising the ferns and their relatives; Bryophyta, the mosses; Lycopodiophyta, which includes the club mosses and some related plants; Marchantiophyta, the liverworts; Anthocerotophyta, the hornworts; and Chlorophyta and Charophyta, two types of green algae. Extinct divisions include the primitive Rhyniophyta, Trimerophyta, and Zosterophyllophyta, among others. The nematophytes were also probably (very early) plants, though their relationships remain somewhat unclear.

The above classification scheme is not universal; some botanists use definitions more or less exclusive than that presented here. According to some classification schemes, red algae and glaucophytes are also considered plants, and Plantae and Archaeplastida completely coincide. (Plants aside from red algae and glaucophytes are then specified as green plants, viridiphytes, or chlorobionts, and given as a group the scientific name Viridiplantae, Viridiphyta, or Chlorobionta.) According to others, however, plants are defined to be multicellular with complex differentiation into tissues, which excludes the green algae. (Outside this classification scheme, such non-algal plants are called embryophytes, metaphytes, or land plants, and placed in the group Embryophyta or Metaphyta.) Sometimes the archaeplastids in general are said to compose Plantae sensu lato (that is, "in a loose sense"), the viridiphytes Plantae sensu strictu ("in a strict sense"), and the metaphytes Plantae sensu strictissimo ("in a very strict sense").

Collectively, the clubmosses, pteridophytes, and magnoliophytes (as well as several extinct groups, including all those mentioned above except the nematophytes) are known as tracheophytes, or vascular plants; the group is sometimes given the scientific name Tracheophyta or Tracheobionta. (The exact taxonomic status of Tracheophyta is undefined; it may perhaps be considered an infrakingdom, but more often it's left without a formal ranking.)

Although fungi are commonly thought of as plants, strictly speaking this is erroneous; they belong to their own kingdom, and are apparently more closely related to animals than they are to plants. Still, on some worlds magical effects geared toward plants affect fungi as well, though this seems to be more a function of the parameters of the arcana than due to any important similarity between the affected organisms.

Anatomy

At the cellular level, all plants have certain characteristic features in common. One of the most obvious and well known is that the cells of plants are surrounded by hard cell walls made of a polysaccharide called cellulose, combined with other organic compounds (most notably hemicellulose, pectin, and often lignin). While plants are not the only Tellurian organisms to possess cell walls, the composition is unique—the cell walls of fungi are made of chitin, and those of bacteria of a polymer called peptidoglycan. The cells are joined by apertures in the cell walls called plasmodesmata. A plant cell also has a large central vacuole surrounded by a membrane called a tonoplast. This vacuole serves several purposes: it stores chemicals, digests proteins, and preserves the cell's rigidity.

Plant cells are also characterized by organelles called plastids, which produce and store various chemical compounds. The best known plastids are the chloroplasts, the sites of the photosynthesis that provides for most plants' energy needs.

Above the cellular level, there are few features that are common to all plants, but many common to certain groups. Moss and vascular plants have specialized tissues to conduct nutrients throughout the organism, called leptome in the former and phloem in the latter. In addition to phloem, vascular plants also have another characteristic tissue called xylem that helps conduct water. Sturdy xylem is a key component of wood, the rigid, fibrous material that makes up the trunks and branches of trees and is harvested as a fuel and building material. Going to an even higher level, most vascular plants, though not necessarily other plants, share certain morphological features: a system of roots below ground, that anchors the plant in place and draws water and minerals from the soil, and stems and leaves above ground, the former making up the plant's structure and the latter catching sunlight for the plant to use in photosynthesis.

Some plants may have a rough analogue to the animal nervous system, in that they are capable of propagating chemical and electrical signals between different parts of their anatomies, and even communicating with each other through hormones. There is some evidence that some vascular plants, through these means, are capable of some limited form of memory and learning. Nevertheless, the popular idea that plants in general have some form of sentience and emotion has not found scientific credence—though certainly there are some non-Euterran plants such as the cai of Doun and the deertree of Jhembaz that do.

Diet

A plant's primary source of nutrition comes from photosynthesis, the use of energy from sunlight to convert carbon dioxide into sugars and other organic compounds. In vascular plants, it is the plants' leaves that catch the sunlight, and where the photosynthesis takes place; other plants may distribute the process throughout the organism. In any case, within the cells, photosynthesis takes place in the chloroplasts, and is primarily mediated through a chemical called chlorophyll. It is the fact that chlorophyll absorbs poorly light in the green wavelengths that gives most plants their green color... although there are other, similar chemicals such as the yellow xanthophylls and orange carotenes that some plants also use. The fact that plants get their nutriment directly from solar energy and don't have to consume other creatures puts them as the bases of many food chains.

Though most plants get by only on sunlight, air, water, and minerals from the soil, some plants do augment their nutritional intake with animal tissue. On True Earth, all these plants are small, their prey being primarily insects and other arthropods, and most of them feed passively, luring insects into traps rather than seeking them out. A few species such as the sundew and Venus flytrap are a little more active, with moving parts to capture their victims, but even they are sessile flora that must lure their victims into reach. On other worlds, however, carnivorous plants exist that go beyond these limitations, possibly feeding on much larger prey, and possessing the ability to chase their prey down rather than only lying in wait for them.

The photosynthetic process involves not only the consumption of carbon dioxide, but also the production of molecular oxygen. This makes for a convenient complement to the respiration of animals, which consume oxygen and exhale carbon dioxide, and makes it possible (with care) to produce an atmospherically closed system of plants and animals that can, in principle, persist indefinitely.

Reproduction

Plants employ a variety of reproductive strategies. Most plants do reproduce sexually, but the exact mechanisms may vary by division. All known plants that reproduce sexually exhibit diplohaplontic alternation of generations, with one generation, the gametophyte, producing diploid gametes that give rise to a generation of sporophytes that produce haploid spores. In turn, the spores grow into (haploid) gametophytes, completing the cycle.

While this general mechanism is more or less universal, the details differ. One notable difference is in which generation—the gametophyte or the sporophyte—is more prominent. In algae, the gametophytes and sporophytes exist as separate organisms, which may or may not resemble each other. In liverworts, mosses, and hornworts, the sporophyte is reduced to a short-lived body grow on the gametophyte, and depend on it for nutrients. In ferns, and club mosses, too, the sporophyte begins by growing on the gametophyte, but the gametophyte (called a prothallus) remains small and usually subterranean, and it is the sporophyte that grows into the larger form that is usually thought of as the plant. The spermatophytes take this still farther, with the female gametophyte developing entirely within the sporophyte, and the male gametophyte comprising only a scant handful of cells within dispersable grains called pollen. Other differences exist within the spermatophyte division; while all spermatophytes begin (their sporophyte phase) as seeds (hence the division's name), in flowering plants these seeds grow in fruit that develop from the flowers that house the plants' reproductive organs, whereas in conifers they develop in woody cones.

Many plants also have the ability to reproduce by budding, growing new copies of themselves from bits of their roots and stems. Some plants artificially bred to have lost their seeds, such as navel oranges and commercial banana cultivars, now are only capable of reproduction by this method.

Origin

Like mitochondria, chloroplasts and other plastids possess their own DNA, leading to the likely conclusion that, like mitochondria, they originated as separate organisms (in this case cyanobacteria) that invaded larger cells and developed mutualistic relationships with them. In effect, even today, plastids could still be considered endosymbionts separate from the plant itself—though it may perhaps be more reasonable to state that they began as endosymbionts but are now so firmly ensconced within their hosts as to jointly form with them a single organism. (Of course, by the hathroic viewpoint, these two statements may not be mutually exclusive.) This relationship began perhaps 1.5 billion years ago, though it isn't entirely clear when arose the line specifically leading to green algae—as opposed to other algae that have different kinds of plastids. In any case, this development can be considered to mark the beginning of Plantae sensu lato, though the beginning of Plantae sensu strictu may be harder to pin down.

Plantae sensu strictissimo seems to have had its origin somewhere about 450 years ago, though the earliest plants are known only from fossilized spores. The earliest plants of which more complete fossils have been found date from the Middle Ordovician, and somewhat resemble modern liverworts. Vascular plants seem to have arisen in the Silurian, though plants didn't grow taller than a few centimeters until the Devonian. By the Late Devonian, however, spermatophytes had entered the scene, and whole forests existed of giant woody pteridophytes. Conifers arrived in the Permian, and flowering plants near the end of the Cretaceous. The fact that the first flowering plants were contemporary to the end of the dinosaurs once led to much theorizing that in fact flowering plants caused the dinosaurs' end, the Terrible Lizards dying off due perhaps to allergies to pollen. In light of other evidence, however, these hypotheses now seem very unlikely—for one thing, the end of the dinosaurs was only a part of a huge mass extinction (the K-T event) that wiped out not just the dinosaurs but about seventy percent of all species then living on Earth, including numerous species of diatoms and other microscopic plankton. More recent discoveries have pointed to the most likely main cause of the K-T extinction event having been a catastrophic asteroid impact, the development of flowering plants not having had anything directly to do with killing the dinosaurs after all.

See also

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