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Dictionary Entries R

Radial symmetry: A term that refers to any living thing that is symmetric along multiple planes going through the center, such as starfish. In plants, an alternative term is actinomorphic. Compare with bilateral symmetry.

Radiation: Energy or particles (in the form of protons or electrons) either emitted or transferred from a particular source. Can also refer to the particles being emitted.

Radioactive: A term used to describe an element which is undergoing radioactive decay.

Radioactive decay: The process where the nucleus of a "parent element" is disintergrating, either spontaneously, or by gaining an electron. The loss of the particle allows the "parent element" to form a sometimes stable "daughter element." The rate of decay for the material is constant and predictable using mathematical equations.

Radiometric Dating: A process of dating geologic material using radioactive isotopes. Some common forms of radiometric dating are ²³⁸U → ²⁰⁶Pb, which has a half-life of about 4.5 billion years, and ⁴⁰K → ⁴⁰Ar, which has a half-life of about 1.25 billion years.

Relative age: A process of determing the age of one geologic material in relation to another. For example, in an undisturbed layered rock formation, the rocks on the bottom of the formation will be relatively older than those at the top.

Relative pollen frequency: A method of comparing the ratio of a species of plant pollen represented in a sediment sample in relation to other species from the same sample.

Reproduction: A process where an organism produces one or more offspring, whether asexually or sexually.

Reservoir: Any body of water that has an artifically controlled flow, like what can be found behind a man-made dam. Can also mean a natural rock formation with a void in which to store liquid material.

Respiration: The process that involves the exchange of oxygen and carbon dioxide between an organism and its environment.

Reticulum: The second stomach in ruminants. The image to the right (courtesy of Seattle PI) shows a simple cartoon of cow digestion, and is labeled as follows:

The cow eats some grass.
The grass is passed onto the rumen and the reticulum where it is softened.
The softened grass is passed back up to the mouth as cud, which the cow chews up and then passes to the third stomach, the omasum.
Nutrients are absorbed in both the omasum and the fourth stomach, the abomasum.
Finally, the stuff leftover is expelled as waste.

Rhizome: In plants, an underground stem that grows horizontally, and can form roots or shoots and can also store food.

Rhizomorph: In fungi, a hyphal strand that resembles and functions similarly to a rhizome.

Ribonucleoside: A nucleoside in which a purine or a pyrimidine base is bound with a ribose molecule.

Ribonucleotide: A nucleotide in which a purine or a pyrimidine is attached to a ribose molecule.

Ribose: A sugar that has five carbon atoms

Ribosome: An organelle made up of proteins and ribosomal RNA. Ribosomes are where protein synthesis happens.

River: A large flowing body of water.

RNA: Ribonucleic acid. A nucleic acid that functions in various forms (such as messengerRNA, and transferRNA) to translate the information in DNA. While DNA is made up of adenine, cytosine, guanine and thymine, RNA is comprised of the first three, but replaces thymine with uracil, which bonds with adenine.

Rock: A naturally occuring solid, comprised of minerals some of which make up the entirety of the rock, and some of which make up a part of the rock. There are three forms of rocks: igneous, metamorphic, and sedimentary rocks.

Root: In plants, on organ which typically grows downward into the soil, and extracts water and nutrients.

Rumen: The first stomach of ruminants. The image to the right (courtesy of Seattle PI) shows a simple cartoon of cow digestion, and is labeled as follows:

The cow eats some grass.
The grass is passed onto the rumen and the reticulum where it is softened.
The softened grass is passed back up to the mouth as cud, which the cow chews up and then passes to the third stomach, the omasum.
Nutrients are absorbed in both the omasum and the fourth stomach, the abomasum.
Finally, the stuff leftover is expelled as waste.

Ruminant: An animal that chews cud, like cows, deer, and goats.