GLOSSARY 



415 



"straight line" evolution being due to natural selection 

 gradually changing with environmental change. 



osmosis the diffusion of a solvent (generally water) through 

 a semipermeable membrane from the side containing a 

 higher concentration of the solvent to the side con- 

 taining a lesser concentration. 



outwash plain a plain usually found beyond the site of a 

 glacier's terminal moraine, formed from glacial debris, 

 and created mostly by deposition from glacial streams. 



ovary (1) in animals, a female sex organ in which eggs 

 are formed and nourished; (2) in ffowerihg plants, the 

 enlarged, basal portion of a pistil that contains ovules 

 or seeds; (3) in any organism, a female sex organ. 



ovipositor an organ in female insects functioning in de- 

 positing eggs. 



ovule an immature seed in a ffower ovary; a structure 

 composed of embryo sac, nucellus, and integuments; 

 when fertilized, it develops into a seed. 



ovum a nonmotile female gamete, an egg. 



ox-bow a fiat to nearly closed, arc-shaped remnant of a 

 former stream bend, formed when the stream cuts be- 

 tween two of its closely approaching bends. 



oxidation a chemical change involving the loss of charged 

 particles in molecules, typically by removing hydrogen 

 or adding oxygen; in biology, a process releasing 

 energy. 



paleogeogrophy the study of past distribution of the 

 hydrosphere, lithosphere, and biosphere or any seg- 

 ment of them. 



paleontology the study of fossils. 



palp an appendage or projecting part of the head, often 

 near the mouth, generally sensory in function. 



papilla any nipplelike structure. 



parallelism evolution of adaptations in a similar direction 

 in different organisms. 



parapodium a flattened, movable appendage occurring in 

 opposite pairs on the body segments of many poly- 

 chaetes (Annelida). 



parasite a symbiote living partly or entirely upon or within 

 an organism (host) and securing part to all of its food 

 from the organism without killing it immediately (or, 

 perhaps, at all). 



parasitism exploitation involving a host and a parasite. 



parenchyma a plant tissue composed of simple or unspe- 

 cialized cells. 



parthenogenesis natural or induced development of an 

 unfertilized egg; "virgin birth. " 



pectoral pertaining to the breast or upper part of the 

 thorax. 



pedlpalp one of the second pair of appendages in the 

 Chelicerata; often sensory, sometimes serving in loco- 

 motion (king crabs), predation (scorpions), or fertili- 

 zation (male spiders). 



pedology the study of soils. 



pellicle a thin, protective, surface layer of certain organ- 

 isms, especially ciliates. 



pelvic pertaining to the posterior limb girdle of verte- 

 brates; the part of the skeleton that supports the hind 

 limbs or fins. 



peneplain an extensive land form of low relief representing 

 the ultimate stage, old age in a cycle of erosion. 



pentactulo the hypothetical adult ancestor of the echino- 

 derm-chordate line of evolution. 



percolation (of water) the movement, mostly downward 

 and laterally, through the fine spaces between rock or 

 soil particles. 



perennial any organism taking more than two years, gen- 

 erally three or more, to complete its life cycle. 



perihelion the point on the orbit of a body in the solar 

 system that is closest to the sun; also applies to any 

 body orbiting about another. 



Period the fundamental and most distinctive unit of 

 geological time, the time during which a rock system 

 was formed. 



periodicity the repeated occurrence of each of the events 

 in the life cycle of an organism or species at frequent 

 and regular intervals. 



periphyton aquatic organisms attached to submerged 

 objects other than the bottom of the body of water. 



permeable penetrable, as a membrane allowing the pas- 

 sage of liquids or gases. 



petiole (1) any stalk; (2) the twiglike part of a leaf. 



petrology the study of rocks. 



pH a symbol denoting the relative concentration of hydro- 

 gen ions in a solution; values range from to 14: 7, 

 neutrality; numbers less than 7, increasing acidity; 

 and numbers greater than 7, increasing alkalinity. 



pharynx roughly, "throat"; the often muscular portion of 

 the digestive tract between the mouth and esophagus; 

 in invertebrates, may contain teeth; in many aquatic 

 vertebrates, the region containing gills. 



phasic cycle a sequence of organisms in a community, 

 starting with the death of an individual and ending 

 with a member of the same species as the first individ- 

 ual. 



phenology the study of periodic phenomena of life and 

 their relations to their environment, mostly weather 

 and climate. 



phloem the plant tissue that functions in food conduction; 

 part of a vein, except in brown algae. 



photoperiodism the response of plants to duration of dark- 

 ness; the basis of long- and short-day plants. 



photosynthesis the autotrophic nutrition process of carbo- 

 hydrate manufacture from carbon dioxide and water 

 that uses light energy and chlorophyll and releases 

 oxygen. 



phylogeny the evolutionary history of a species or larger 

 taxon. 



