Glossary - 693 



cate microscopic and ultramicroscopic structure 

 and certain characteristic activities; i.e., metabol- 

 ism, liberation of energy as movement, heat, etc., 

 responses to stimuli; growth and reproduction. 



Pseudopodia. Temporary projections of the proto- 

 plasm of a cell, effecting locomotion of the cell and 

 phagocytosis. 



Ptyalin. An enzvme secreted by the salivary glands, 

 which partially hydrolyzes starch. Also called sali- 

 vary amylase. 



Pupa. A quiescent stage of many insects, between 

 the larval and adult stages. 



Radial Symmetry. Svmmetry built around a line. 

 In a radially symmetrical body (e.g., a cylinder) , 

 any plane that passes through the line of the cen- 

 tral axis will divide the body into symmetrical 

 halves. 



Radiant energy. Energy transmitted through 



"empty" space (i.e., space devoid of matter) ; in- 

 cluding light, x-rays, etc. 



Radical. A group of atoms that acts as a unit in 

 many chemical reactions. 



Radicle. Tne root of an embryo plant within the 

 seed. 



Receptacle. The base of a flower, to which all other 

 parts of the flower are attached. 



Receptor. A structure specialized for the reception 

 of some particular kind of stimulus. 



Recessive. A gene whose phenotypic effect is largely 

 or entirely obscured by that of its allelomorph. 

 (cf. Dominant.) 



Recombination. A new combination of genes 

 brought about by the normal processes of crossing 

 over, reduction, and fertilization. 



Rectum. The terminal portion of the enteron. 



Reduction. Chemical reaction in which hydrogen is 

 accepted or oxygen is yielded up by a substance. 

 Also the acceptance of electrons by a substance. In 

 genetics, the change of chromosome number from 

 diploid to haploid. 



Reflex. A response to a particular stimulus; a reflex 

 may be simple or complex, unconditioned or con- 

 ditioned. When not otherwise qualified the term 

 reflex usually means a simple unconditioned re- 

 sponse; i.e., one involving only a single action and 

 independent of any previous experience of the 

 individual, (cf. Conditioned reflex, Habit, In- 

 stinct.) 



Reflex arc. A series of neurons transmitting excita- 

 tion successively from a receptor through the cen- 

 tral nervous system to an effector; comprising one 

 sensory and one motor neuron with or without one 

 or more intermediate neurons. 



Refractory state. The state of a cell for a brief 

 period immediately following excitation, during 

 which it cannot be excited again by another stimu- 

 lus. 



Reproduction. The production of organisms from 

 those already living. 



Respiration. The intake of oxygen and elimination 

 of carbon dioxide. (Often used as including oxida- 

 tion in the cells.) 



Response. Any change of activity resulting from a 



change of external or internal conditions, (cf. 

 Stimulus.) 



Responsiveness. The property of responding to 

 changes of conditions by changes of activity. 



Resting stage. The intermitotic, or growth, stage of 

 the cell. 



Rhizoid. A rootlike organ of simple structure; dis- 

 tinguished from a true root by not containing vas- 

 cular bundles. 



Ribosomes. Numerous fine granules seen bordering 

 the endoplasmic reticulum in electronmicrographs 

 of cells generally; rich in RNA; associated with the 

 the synthesis of enzymes and other proteins. 



RNA. Ribonucleic acid, in which the sugar of the 

 sugar-phosphate chain is ribose; occurs in three 

 fractions: (1) ribosomal, (2) messenger, and (3) 

 transfer RNA, all concerned with protein synthesis. 



Root hairs. Filamentous projections of the epider- 

 mal cells of roots shortly above the growing point. 



Salivary glands. Glands located near the mouth, 

 which secrete saliva into the mouth. 



Salt. A substance that produces positive ions other 

 than H + and negative ions other than OH - . 



Sap. The watery solution of salts, etc., in the ducts 

 and tracheids of a plant. 



Saprophytic. A mode of nutrition in which the or- 

 ganism absorbs organic food directly from the en- 

 vironment. 



Sarcoplasm. The liquid portion of the cytoplasm 

 of a muscle cell, between the myofibrils and fila- 

 ments. 



Secretin. An internal secretion of the duodenum, 

 which stimulates the pancreas to secrete pancreatic 

 juice. 



Secretion. The liberation from cells of substances 

 that have been synthesized or accumulated in these 

 cells. Any substance or mixture thus secreted. 

 (Often used as including also the synthesis of such 

 substances in the cells.) 



Sedimentary rocks. Rocks formed by the deposition 

 in water of sand, clay, mud, etc., which subse- 

 quently hardens. 



Seed. An embryo sporophyte surrounded by the re- 

 mains (if any) of the mother gametophyte (endo- 

 sperm) and external integuments derived from the 

 ovule of parent sporophyte. 



Segregation. The separation of the two genes of an 

 allelomorphic pair into different germ cells during 

 meiosis. 



Selection. Any process tending to favor the preserva- 

 tion and reproduction of one type of organism 

 rather than another. 



Self-fertilization. Union of gametes both derived 

 from the same individual, (cf. Cross-fertilization.) 



Semipermeable membrane. A membrane through 

 which some, but not all, of the components of the 

 separated solutions can diffuse. 



Sense organ. A multicellular receptor organ, includ- 

 ing sensory cells and accessory structures associated 

 with them. 



Sensory cell. A unicellular receptor. 



Sensory neuron. A neuron that either acts as a 

 receptor itself or receives excitation directly from 



