692 - Appendix 2 



compounds that dissociate forming hydroxyl 

 (OH-) ions. 



Organic compounds. Compounds of carbon and hy- 

 drogen in which some (or, rarely, all) of the hydro- 

 gen may be replaced by other elements or radicals. 



Organism. An individual living thing. 



Osmosis. Exchange of solvent (usually water) be- 

 tween two solutions separated by a semipermeable 

 membrane. 



Ovary. The female gonad of animals, in which the 

 egg cells are formed. 



Oviducts. The female gonoducts. 



Oviparous. Laying eggs, either before or shortly after 

 they are fertilized, (cf. Viviparous.) 



Ovule. The macrosporangium of seed plants. 



Ovum. The egg cell. 



Oxidation. A chemical reaction in which oxygen is 

 accepted or hydrogen is yielded up by a substance. 

 Also includes yielding of one or more electrons by 

 a substance. 



Pancreas. An important digestive gland of verte- 

 brates. 



Parasite. An organism that lives on or in another 

 living organism, from which it derives nourish- 

 ment. 



Parasympathetic system. That portion of the auto- 

 nomic nervous system which is connected with the 

 brain and the posterior end of the spinal cord, and 

 whose ganglia lie in or close to the organs they 

 innervate. 



Parenchyma. A relatively undifferentiated tissue. 



Parthenogenesis. Development of an egg cell with- 

 out fertilization. 



Pellicle. A thin, flexible external cell membrane, 

 outside the plasma membrane; characteristic of ani- 

 mal cells. 



Pepsin. An enzune secreted by the stomach, which 

 partially hydrolyzes proteins. 



Peristalsis. Rhythmic contraction of the wall of the 

 enteron or other tubular organs, traveling in suc- 

 cessive waves in one direction. 



Peritoneum. The membrane lining the abdominal 

 cavity. 



Petals. The separate parts of the corolla. 



Petiole. The stalk of a leaf. 



Phagocytosis. The ingestion of solid particles by 

 certain of the cells of a multicellular organism. 



Pharynx. The portion of the enteron of vertebrate 

 animals, immediately behind the mouth cavity. 



Phenotype. A type of organism judged on the basis 

 of its visible characteristics, (cf. Genotype.) 



Phloem. That portion of the fibrovascular bundles 

 which contains the sieve tubes, usually the outer 

 portion of the bundles, (cf. Xylem.) 



Phospholipids. Lipids containing phosphorus and 

 nitrogen, found in all cells; essential components of 

 protoplasm. 



Photoperiodism. Processes by which the flowering 

 and other responses of plants are regulated by sea- 

 sonal (and experimental) changes in exposure to 

 light and dark. 



Photosynthesis. The formation of carbohydrates 

 and other compounds from carbon dioxide and 



water by the absorption of radiant energy of sun- 

 light by chlorophyll. 



Phototropism. Movement determined by the direc- 

 tion of incident light. 



Phyloceny. The line of descent of a major group of 

 organisms. 



Phylum. A taxonomic group, next below a kingdom. 



Phytohormones. Auxins and other plant hormones. 



Pistil. A separate carpel or a group of carpels united 

 to form a single compound organ. 



Pith. The parenchyma in the center of a stem or 

 root. 



Pituitary Gland. A ductless gland on the ventral 

 side of the brain, above the roof of the mouth, 

 composed of two lobes having different origins and 

 different secretions. 



Placenta. An organ of most mammals, by which the 

 embryo is attached to the wall of the uterus and 

 through which exchanges occur between the blood 

 of the embryo and that of the mother. 



Plasma membrane. The differentiated film at the 

 outer surface of the cytoplasm, of ultramicroscopic 

 thickness. 



Plastids. Cytoplasmic formed bodies in which occur 

 special constructive metabolic processes. 



Polar bodies. The very small, functionless cells that 

 are produced, with the mature egg cell, by the mei- 

 otic divisions of the oocyte. 



Pollen grain. The microspore of seed plants. 



Pollen tube. The male gametophyte of seed plants. 



Pollination. The transfer of pollen grains to the 

 stigma. 



Polyembryony. The production of two or more em- 

 bryos from one, by fission. 



Polysaccharide. A carbohydrate, each molecule of 

 which yields, by hydrolysis, many monosaccharide 

 molecules. 



Pons. A region on the ventral side of the hind part 

 of the brain; an important intermediate station be- 

 tween the cerebrum and the cerebellum. 



Population. Any group of individuals, considered as 

 a whole, especially with reference to numbers and 

 statistics. 



Portal vein. A vein that starts and ends in capil- 

 laries. 



Potential energy. Energy inherent in the position 

 or configuration (relative positions of component 

 parts) of a body. (cf. Kinetic energy.) 



Prophases. The early stages of mitosis or meiosis, 

 preceding metaphase. 



Proprioceptors. Special receptors for internal stim- 

 uli due to the positions and movements of the parts 

 of the body. 



Proteins. Complex organic compounds of C, H, O, 

 N, and often other elements, Yielding amino acids 

 by hydrolysis; essential components of protoplasm. 



Prothallium. The gametophyte of vascular plants, 

 especially ferns. 



Protonema. An early stage in the development of 

 the gametophyte in mosses. 



Proton. A particle of positive electricity, one of the 

 particulate constitutents of matter. 



Protoplasm. Living matter, a complex system of 

 organic and inorganic substances, having an intri- 



