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GLOSSARY 



multicellular many celled. 



mutation an abrupt but stable change in a gene or chromo- 

 some that causes hereditary modification in an in- 

 dividual. 



mutualism symbiosis in which both participating or- 

 ganisms benefit and require the relationship. 



mycelium a mass of hyphae. 



natural group an assemblage of different species 

 recognized by being the only descendants from a 

 common ancestor; a monophylletic group. 



natural history the study of all aspects of the universe 

 except those pertaining directly only to man; particu- 

 larly, a study of things as they exist in nature. 



natural selection the tendency in evolution for most 

 organisms possessing certain characters in a given en- 

 vironment to produce many more offspring than do 

 those lacking such characters; roughly, the process 

 tending toward survival of the fittest. 



nekton aquatic, swimming organisms that actively direct 

 their movements and are free of the bottom. 



nephridium a tubular excretory organ of invertebrates, es- 

 pecially annelids, arthropods, and mollusks. 



nerve a bundle of nerve cell fibers occurring outside the 

 brain and spinal cord. 



nerve cord a group of nerve cells and often ganglia; with 

 the brain they form the central nervous system. 



neuston aquatic organisms that float against or are closely 

 associated with the surface film of water. 



neutralism hypothetical symbiosis in which the organisms 

 involved neither benefit nor harm one another. 



niche (1 ) ecological or functional, the status or role of an 

 organism in its environment; its activity and relation- 

 ships in its environment; (2) habitat or place, the 

 microhabitat or specific area occupied by an organism. 



nocturnal pertaining to night or hours of darkness; neither 

 diurnal nor crepuscular. 



node the point on a plant stem from which one or more 

 leaves arise; buds often arise here at the angle between 

 the stem and upper surface of a leaf. 



nomad an animal that wanders from one locale to another 

 without having a fixed dwelling place. 



nomenclature the process and procedure for applying 

 scientific names to organisms. 



notochord a longitudinal, internal, elastic, skeletal rod of 

 cells below the nerve cord and found in all embryos and 

 some adult chordates; in most vertebrates it is com- 

 pletely replaced by the backbone or vertebral column. 



nucellus a plant tissue in a young ovule; probably repre- 

 sents the female sporangium wall, is located within 

 the integument, and encloses the site of embryo sac 

 development. 



nucleic add an organic acid composed of repeated units of 

 nucleotides (each composed of a sugar phosphate 

 joined to a nitrogenous base); two main types, DNA 



thecarrierof the genetic information and RNA (which 

 is controlled by DNA) the director of metabolic 

 activities. 



nucleus a structure within all cells and cell-like organisms 

 except the Monera; designated by an outer membrane, 

 internal sap, and chromosomes. 



nutrition the total processes in an organism that involve 

 taking in and using food for maintenance, growth, 

 repair, and energy production. 



nymph an immature stage of any insect displaying simple 

 metamorphosis; resembles the adult, but differs mostly 

 in body proportions (e.g., the wings are abbreviated or 

 absent) and perhaps the addition of structures in 

 aquatic forms. 



oceanography the study of the geography of the sea; some- 

 times extended to the study of all aspects of the sea. 



oligotrophic pertaining to substrates, especially water, 

 poor in nutrients, especially organic nutrients or 

 sources of nutrients for plants. 



omnivore an organism, generally a bulk feeder, that does 

 not feed upon a single type of food or host; generally, 

 a combination carnivore and herbivore. 



operculum a lid; in mosses, the lid covering the capsule; 

 in some snails, the lid covering the shell opening; in 

 bony fishes and chimaeras, the lid covering the gills. 



opposite leaves or buds those occurring in pairs at a node. 



oral pertaining to or near the mouth. 



order a taxon between class and family; a large category 

 of soils that includes one or more families. 



organ a group of tissues and perhaps some cells that act 

 as a unit to perform one or more functions. 



organ system a group of organs acting as a unit to perform 

 a definite function or set of associated functions (e.g., 

 the digestive system, which accomplishes digestion and 

 related functions); the only organ system in plants are 

 flowers and related structures like certain cones. 



organelle any specialized protozoan structure that, in 

 performing a specific function, resembles an organ's 

 function. 



organic pertaining to, coming from, or having unique 

 features of living organisms. 



organic horizon or layer the layer of recognizable organism 

 remains and of underlying humus that occurs above 

 soil. 



organism a single living creature, generally restricted to 

 individuals within the Monera, Protista, Plantac, and 

 Animalia, but excluding viruses and nonliving things. 



orthogenesis "straight line" evolution; the trend in some 

 evolution for organisms to follow a particular direction 

 for some time; now often called orthoselection (q.v.). 



orthoselection orthogenesis explained in terms of mutation 

 and natural selection during a long period of a some- 

 what continuous amount of change in environment, the 



