olfactory (ohl-fac'to-ree) lobes: most anterior part 

 of the vertebrate brain, containing centers of 

 smell. 



opsonin (op'soh-nin) : antibody in normal or im- 

 mune blood serum which makes a particular 

 kind of invading germ more easily eaten by 

 phagocytes. 



optic lobes: in vertebrates below mammals two 

 distinct spherical portions of the brain be- 

 hind the cerebrum, containing the centers of 

 sight ; in mammals optic lobes are small and 

 inconspicuous. 



order: in classification the largest subdivision 

 within a class. 



organ: distinct part of the body of a plant or ani- 

 mal which consists of tissues working to- 

 gether to carry on some activity. 



organic compound: one of a large number of com- 

 plex carbon compounds; often obtained from 

 the bodies of living things or made by man. 



organic disease: disease caused by the improper 

 working of some particular organ. 



organism: any single living thing. 



ornithology (or-nith-ol'o-jee) : study of birds. 



osmosis (oss-moh'sis) : technically the diffusion 

 of water through a membrane; also used for 

 diffusion of dissolved substances through a 

 membrane. 



outbreeding: see crossbreeding. 



ovary (oh'va-ree) : in many-celled animals the or- 

 gan in which the eggs or ova develop. In 

 higher forms special sex hormones are also 

 produced in the ovary. In seed plants the low- 

 est part of the pistil in which ovules with 

 eggs develop. 



overproduction: the reproduction by animals and 

 plants of more offspring than can survive. 

 Darwin pointed out that this results in a 

 struggle for existence. 



oviduct (oh'vee-duct) : tube from the ovary 

 through which eggs pass toward the exterior. 



ovule (oh'vule) : in seed plants that structure 

 which normally grows into the seed after fer- 

 tilization; in the true flowering plants it lies 

 within the ovary and contains the embryo sac 

 with its egg cell. 



ovum (oh'vum) ; pi. ova: an egg cell. 



oxidation: chemical union of oxygen with some 

 other substance; results in the release of en- 

 ergy. 



Pi generation: parental generation; first two in- 

 dividuals crossed in any given breeding ex- 

 periment. 



Paleolithic (pay-lee-oh-lith'ic) Age: see Old Stone 

 Age. 



paleontology (pay-lee-on-tol'o-jee) : study of the 

 organisms that lived in former ages. 



palisade cells: in a green leaf the regular, upright 

 cells just under the upper epidermis. 



pancreas (pan'cree-as) : digestive gland near the 

 stomach, which pours its juice into the upper 

 end of the small intestine; other cells in the 



pancreas are ductless gland cells secreting 

 insulin into the blood. 



parasite (par'a-site) : plant or animal that lives 

 in or on some other living organism (its 

 host), taking its food from the host. 



parathyroids (par-a-thy'roids) : two pairs of small 

 ductless glands lying close to the thyroid; 

 their secretion regulates the assimilation of 

 calcium by the body. 



parenchyma (per-en'kim-ma) : tissue in plants 

 that consists of thin-walled cells with large 

 vacuoles; it may or may not have chloro- 

 plasts. 



parthenogenesis (parth-en-oh-jen'e-sis) : develop- 

 ment of an egg without fertilization, as in 

 some insects and other animals. 



pasteurization (pas-ture-iz-ay'shun) : heating a 

 liquid, such as milk, to a temperature of 

 about 150 degrees F., followed by chilling, 

 so as to kill nonspore-forming bacteria. 



pathogenic (path-oh-jen'ik) : causing disease; 

 term applied to certain microorganisms. 



peat: partly decomposed vegetable matter found 

 in marshy regions; used as fuel when dried. 



pedigree (ped'ih-gree) : table or chart showing 

 the line of ancestors of a person or some 

 other organism; record of family history. 



pellagra ( peh-lah'gra ) : deficiency disease caused 

 by insufficient intake of niacin. 



penicillin (pen-i-sill'in) : a powerful antibiotic 

 made by the mold Penicillium; used in treat- 

 ing certain diseases. 



pepsin (pep'sin) : enzyme in gastric juice which 

 in the presence of hydrochloric acid changes 

 protein into smaller molecules such as pep- 

 tones. 



peptones (pep'tones) : diffusible, soluble sub- 

 stances into which proteins are changed by 

 enzymes such as pepsin; intermediate prod- 

 ucts of digestion. 



perennial ( peh-ren'ee-el) : plant that normally 

 lives more than two years. 



period: geologic subdivision of time within an 

 era. 



peristalsis (perr-i-stali'sis) : wavelike contractions 

 of the rings of muscle in a tubular organ in 

 an animal; in man food is pushed along the 

 alimentary canal in this way. 



petals (pet'els) : leaflike parts of a flower, usu- 

 ally white or brightly colored, lying within 

 the sepals. 



petiole (pet'ee-ohl) : stalk of a leaf which at- 

 taches it to the stem of the plant. 



petrifaction (pet-rih-fak'shun) : process by which 

 parts of living things are turned to stone. 



phagocyte (fag'o-site) : one of the white blood 

 cells that takes in and destroys bacteria and 

 other foreign particles. 



phloem (flow'em) : outer portion of the vascular 

 cylinder in the roots and stems of ferns and 

 seed plants; contains food-conducting cells 

 (sieve tubes) and fibers. 



593 



