susandragon's New Writeups

geophyte (thing)

2002-06-30T11:24:03Z

A geophyte is a plant that depends for part of its life cycle on an underground reserve of food, that is a bulb, corm, rhizome, or tuber. In adverse conditions, such as the cold of winter, the above-ground growth of the plant can die back and the geophyte enters its so-called dormant phase, though below ground it is still active, not dormant.

The storage organ is often referred to as a bulb, regardless of its exact shape and which tissues it is derived from, but the true bulb of the lily, onion, hyacinth, daffodil, tulip and so on is formed from modified leaves around stem tissue. The leaves, called scales, are the storage medium.

A corm is where the stem itself is modified: examples are the crocus and gladiolus.

Both corms and bulbs arise upward out of a compacted base of stem material called a basal plate. Tubers and rhizomes are also made from stem material, but grow in different ways: a tuber goes downward, like a potato, and a…

vraic (thing)

2002-05-18T09:15:44Z

A Channel Islands word for seaweed, which they gather for use as fuel and manure. It is a dialectal form of the French varec, which is also used in English, and they both come from Scandinavian words related to the English wrack, a kind of seaweed so called from being wrecked or driven onto shore.

intertidal (place)

2002-05-18T08:49:15Z

The coastal zone that is sometimes wet and sometimes exposed to air because of the action of the tides. The intertidal zone is also called the littoral zone. Because of the extreme variability of conditions on a daily basis, creatures of the intertidal zone are well adapted to these stresses, and there is a vertical differentiation of the zone into sub-zones based on how much water they are usually exposed to.

The high intertidal is inhabited by creatures such as crabs, limpets, and barnacles, which are happy to be completely dry for a large part of the day. In the middle region are the creatures typically found inside rock pools, such as sea anemones, mussels, and starfish. They need regular immersion but have enough resources to survive the periods without. At the lowest level are essentially marine creatures like sea urchins which have adapted to survive some degree of exposure.

Above the high tide mark is the supralittoral or spray zone. This…

Phaeophyta (thing)

2002-05-18T08:08:30Z

Phaeophyta are the brown seaweeds. They are not plants, and genetic analysis shows they should be treated as a separate kingdom within the domain Eucarya. Nor are they closely related to red seaweeds, which are the kingdom of Rhodophyta. Green seaweeds are plants.

Several thousand* species are known, including kelp and wrack and other familiar large tough seaweeds washed up on beaches. Some are edible. Some are giants, and can reach 100 m long and form huge forests under the sea. The Sargasso Sea in the North Atlantic is dominated by a giant floating forest of Sargassum. But most species are anchored.

Their brown pigment is a xanthophyll called fucoxanthin. The characteristic long blade of the familiar species is called the thallus, and the fist-like lump that anchors them is called the holdfast. The trunk from which the thalli radiate is the stipe. The large bladders enable them to float but there may also be sexual sacs.

They have both…

synapomorphy (idea)

2002-05-15T20:45:21Z

In biology, an apomorphy is a characteristic of an organism that is not shared by a larger group it belongs to, nor by the common ancestor of the larger group. So it is an innovation, evolved after the organism's line diverged from the group ancestor.

A synapomorphy is an apomorphy that is shared by more than one organism. It indicates that the organisms that share it are more closely related, and they had a common ancestor which innovated the apomorphy.

Synapomorphies are the features used in strict taxonomy to set up clades, or groups with common ancestry.

For example, birds are generally distinguished by feathers and flight, and (probably) derive from dinosaurs which had neither. The placenta is the innovation that marks placental mammals out from other mammals. These features are synapomorphic to those groups.

The notion of synapomorphy is opposed to homoplasy, which is a feature in common because of convergent evolution. Birds, bats,…

evaporite (thing)

2002-05-09T20:25:11Z

A general term for minerals deposited as sedimentary layers by the evaporation of water. Examples are gypsum, anhydrite, and rock salt or halite.

They may be markers of areas of commercial interest, because some evaporites are both non-porous and resilient (they deform rather than break), so hydrocarbon deposits are effectively capped by them.

Since salt water is heavier than fresh water, evaporation from a salt sea causes the salts to precipitate to the bottom.

Study of salts and carbonates in martian rocks deposited on Earth as meteorites, and which appear to be evaporites, may reveal the history of aqueous activity on the surface of Mars.

Useful diagrams at www.geo.msu.edu/geo333/evaporite.html