User:JonRichfield/Apod

Apod, apodous and apode are morphological terms variously derived from the same root; literally they mean “without feet” or "footless", but in practice the applications of the terms may be literal, figurative or both. The terms are broadly, even vaguely, applied in biology, because the concept has implications in a wide variety of adaptive contexts.

Etymology and Definitions

Apod and related words come from Greek a — pous and its Late Latin derivations. The uses of the word and its derivations are arbitrary; they have been adopted both by zoologists and by botanists in assorted senses and connections, both figurative and literal. As instanced below, analogies between many of the applications of the term are tenuous, so in general it is best to read them in context as meaning "footless" or "with the physical base or stem drastically reduced or absent", and to not to assume any uniform interpretation.

Apod- occurs as a component in various biological names and terms. In zoology the terms are "applied to birds, fish, and reptiles, in which feet or ventral fins are either absent or only rudimentary". However, discretion is necessary; several Greek roots begin with the equivalent of apod other than those that mean "footless". So for instance apodeme is from a different root, literally meaning "away from home (or house)"; it describes an internal projection onto which muscles or tendons are attached "away from" the external body wall.

Derivations of Apod- in the sense of "footless" include:

* Nouns apod and apode, meaning "legless creature", and apody or apodia for the condition of footlessness or leglessness.

* Adjectives apod,apodal, apodial, or apodous, meaning "without legs or feet", or in botany, "without a podium or foot-stalk".

Such derivations, together with assorted adverbs, have been coined ad hoc for centuries and several of the words commonly are used indifferently as nouns or adjectives; the details matter little as long as the sense is clear.

Biological significance

In spite of the vagueness of the term, there are some important biological principles associated with various conditions referred to as apodal, whether in botany or zoology.

Botany

Pinus palustris needles and cones

Larix decidua needles

Acacia sieberiana foliage — petiolate pinnate leaves with apodous leaflets

A Gasteria with a typically apodous habit

Tillandsia fasciculata is an apodous epiphytic Bromeliad

In botany sessile leaves are sometimes called apodous, and that usually is a condition in which leaves have broad bases and therefore can be very firmly seated. On the other hand such attachment limits the ability such leaves to adjust their position and growth to make the most of the available light. There also may be complex implications for survival in very windy conditions; for example, the leaves cannot thrash harmfully in a wind, but then they also cannot bend to shed wind either for their own protection or to reduce wind stress on the trunk.

Forest trees for example are largely sheltered from severe wind stresses. Their leaves are not subject to much wind shredding so they commonly have leaves with firm petioles. Trees adapted to open spaces and high winds, such as some willows and palms often have supple petioles and leaves shaped to shed the wind. Other trees have adapted to such stresses with highly compound leaves. Examples include some species Acacias such as Acacia sieberiana. Such leaves are in a sense "pre-shredded" and although their leaves have supple petioles their leaflets are in fact apodous. In direct contrast to forest trees heathland shrubs do not grow as high, generally have no shortage of light, often suffer a shortage of water and a high exposure to wind; they usually grow small, compact apodous leaves embracing gracile stems. Such a habit is so characteristic that, although it occurs in many plant families, it is called the ericoid habit.

Many conifers have apodous leaves, and in particular in the Pinaceae the leaves have been extended into acicular "needles" without obvious petioles or blades. Bladeless leaves are not very vulnerable to wind or accumulation of snow. They may be supple and many centimetres long, as in Pinus palustrus, adapted to temperate climates, or just a few centimetres long, as in Larix species that largely inhabit cold temperate to sub-frigid zones. Acicular foliage is an effective means of shedding wind and snow, but still may be fairly well adapted to forest conditions as long as the needles are sufficiently densely spaced; accordingly the needle-bearing conifers tend to dominate in far northerly forests, whereas broadleaved trees usually dominate in temperate zones, with mixed forest in intermediate regions.

Viscum minimum, an apodous parasitic growth on a Euphorbia, most of the parasite invisible within the host plant.

When it happens that their communities are successfully invaded by forest, shrubland and savanna plants with apodous leaves generally do badly because their sclerophylic habit is not well adapted to the light levels under forest canopy. Usually they soon are replaced by shade-adapted undergrowth which generally has petiolate leaves, well adapted to capturing any available light, but not suited to resisting either drought or strong wind.

The term "apodous plants" refers to those with little or no above-ground stem. As a rule this means that they are geophytes arising from underground structures such as bulbs and corms. Some plants however, are called apodous because they have very little stem, and not much of that is visible above the ground. This is a condition found in many succulents, such as Gasteria but also in thicket plants and epiphytes such as many Bromeliaceae, and in parasitic plants such as many species of mistletoe.

Haemanthus sanguineus has flat, apodous leaves that suppress competing seedlings

Giant tortoise on natural turf

Apody of leaves or inflorescences also is common among plants adapted to resisting grazing pressure. Illustrative examples occur on some islands heavily grazed by giant tortoises, such as Aldabra. Grazing pressures have led to the establishment of "tortoise turf", a community of species of grasses, sedges, and forbs, not generally related, but with similar adaptations to the grazing. The plants tend to be genetically dwarfed and their inflorescences grow apodially, not from the tops of the plants, but at ground level, even though the grasses and sedges are wind pollinated, and wind pollination works best when the flowers are held high off the ground.

Some apodous plants with broad, flat leaves, such as Haemanthus sanguineus actually use their large deciduous, apodous, flat leaves, supported by very large subterranean bulbs, to outcompete neighbouring annuals and turf plants; they lay the leaves flat on the ground, and thereby deny light to the plants beneath. Their seedlings in contrast are too small to support such a strategy; instead they annually produce strap-shaped narrow leaves for a few years until they grow large enough to apply the domination strategy.

Certain types of Tumbleweeds represent an interesting class of botanical apody. Some are largely apodous plants or have apodous inflorescences. In either case their bulk outline is sufficiently convex for them to roll when blown by the wind. After setting and maturing their seed, the tumbleweed part dries and becomes light and springy. Then either the inflorescence breaks off from the main body of the plant, as in Crossyne, or the whole plant breaks off from the root base. The released structure rolls in the wind, progressively disintegrating and shedding seeds as it goes. The effect is commonest in open, usually arid, country, which makes tumbleweeds a conspicuous feature in their dispersive season. They consequently have become something of a symbol or perhaps a cliché in Western films; in Giant for example, tumbling tumbleweeds set the tone when the newlywed Benedicts return to Texas. [1]

Zoology

Conditions referred to as apodous probably occur in far wider forms and contexts in zoology than in botany. From a trivial point of view one could regard animals such as worms, or worm-like molluscs such as Teredo as apodous, but in such a context that usage is rare.

More commonly one might use the term in contrasting apodous animals with genetic relatives or with phases of metamorphosis in which legs do occur. For example, the tadpoles, or apodous larvae, of most extant amphibians, grow into legged adults. Again, apodous eruciform beetles of beetles in the families Curculionidae and Cerambycidae have well developed legs when they have metamorphosed into adults, and they also contrast with the eruciform larvae of the Symphyta and the Lepidoptera, that have functional legs from the time they leave the egg.

Interestingly, the adult females of many bagworms, such as the wattle bagworm, in contrast to the males, are at once apodous, wingless, and blind, in fact rather wormlike, far more so than they were in their eruciform larval instars, which are not apodous. The apodous adult female has no need for locomotion beyond turning round in her bag, once to be mated, and once again to lay eggs.

In bygone days biologists referred to adaptations to loss of function by such terms as “degenerate forms”. In modern usage however, biological degeneracy has a very different meaning; it concerns biological complexity, and not loss of function. The old sense of loss of functions, such as in many advanced parasites, being a trend towards inferiority, is now seen as a value-laden, often naïve, misrepresentation of adaptation. Examples are no longer easy to find in textbooks. Seen as an adaptation, loss of functions that are no longer valuable, and might even be hazardous and resource-wasting, apody for example, is as much an adaptive effect of natural selection as any other, so that the pejorative label of “degeneracy” would make little sense.

In contrast some lizards of the families Anguidae (e.g. genera ‘’Anguis’’ and ‘’Ophisaurus’’) and Scincidae (e.g. genera ‘’Typhlosaurus’’ and ‘’Acontias’’) are completely apodous all their independent lives, whereas several other genera in the Scincidae, such as ‘’Lygosoma’’ have distinctly short legs, and members of the genus ‘’Scelotes’’ range from species with four small, but well-developed legs (such as ‘’Scelotes mira’’), through species with reduced numbers of toes (e.g. Scelotes caffer), species with hind limbs only (e.g. Scelotes bipes) to completely apodous species such as Scelotes anguina.

There is a strong tendency for such lizards to have burrowing lifestyles, and there are grounds for suspicion that even non-burrowing extant species descended from burrowing ancestors. This also is believed to apply to modern snakes; they presumably descended from burrowing ancestors and then re-emerged and regained part of the function of their eyes, but not of their legs, except for vestigial claws and pelvic girdles in most of the Pythonidae and Boidae and pelvic girdles in the Typhlopidae and Leptotyphlopidae. Those that have re-emerged from a burrowing niche into the upper world, have all adopted other niches suited to their morphology in one way or another. Some inhabit low-growing dense vegetation such as grasses, some are more or less arborial, and some, mainly the sea snakes (Hydrophiidae), have become partly or completely aquatic. All of these niches, as well as burrowing in suitable soils or passages such as those in termite nests, are well-suited to efficient, stealthy locomotion by a sinuous flexing of the body. The apodous morphology also lends itself to constriction of prey.

A beautiful example of an emergent adaptation of apody to efficient locomotion is sidewinding, which occurs mainly in certain arenicolous species, mainly members of the family Viperidae, the vipers and Crotalids. Many snakes progress in some sort of lateral undulation when forced to move over a smooth surface or swim. Sidewinding is a subtle and confusing development in which the snake lifts up to three parts of its body, typically its head, tail and middle, and places them down without much disturbing the sand. It then follows each of the three contacts with the immediately adjacent parts of the body, in a smooth, efficient, and surprisingly rapid motion, leaving only a series of S-shaped tracks in the sand, with the marks of the ventral scales still visible to show that there had been practically no lateral movement on the surface. Only a few species of snakes normally use the motion, mainly one rattlesnake, the sidewinder Crotalus cerastes, and a few species of African viper, such as Bitis peringueyi. Interestingly however, unrelated snakes of the Colubrid subfamily Homalopsinae, use the same principle for locomotion across tidal mudflats.

In animals such as insect larvae, loss of limbs generally accompanies a lifestyle in which legs have little relevance. However, the range of niches and adaptations, not surprisingly, is much wider than among vertebrates.

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Linnaeus was a professor at the University of Uppsala and an eminent botanist; however, one of his colleagues, Peter Artedi, earned the title "father of ichthyology" through his indispensable advancements. Artedi contributed to Linnaeus's refinement of the principles of taxonomy. Furthermore, he recognized five additional orders of fish: Malacopterygii, Acanthopterygii, Branchiostegi, Chondropterygii, and Plagiuri. Artedi developed standard methods for making counts and measurements of anatomical features that are modernly exploited. Another associate of Linnaeus, Albertus Seba, was a prosperous pharmacist from Amsterdam. Seba assembled a cabinet, or collection, of fish. He invited Artedi to utilize this assortment of fish; unfortunately, in 1735, Artedi fell into an Amsterdam canal and drowned at the age of 30. Linnaeus posthumously published Artedi's manuscripts as Ichthyologia, sive Opera Omnia de Piscibus (1738). His refinement of taxonomy was culminated subsequent to the development of the binomial nomenclature which is in use by contemporary ichthyologists. Furthermore, he revised the orders introduced by Artedi, placing significance on pelvic fins. Fish lacking this appendage were placed within the order Apodes; fish containing abdominal, thoracic, or jugular pelvic fins were termed Abdominales, Thoracici, and Jugulares respectively.

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