APPIIKIODX 

 Glossary 



THIS LIST dues not include names of taxonomic 

 groups, which are defined in Appendix 1, nor most 

 terms that are used only once in the text and defined 

 there. 



ABERRATION. An irregularity of chromosome be- 

 havior, resulting in an unusual chromosome com- 

 plex. 



Absorption. The passage of water and dissolved sub- 

 stances into the cell or the organism. 



Acid. A substance that dissociates, yielding hydro- 

 gen (H + ) ions (but not hydroxyl (OH - ) ions). 



Acquired characters. Somatic modifications; i.e., 

 variations caused by environmental factors. 



Acrosome. Specialized structure found at anterior 

 tip of the head of a sperm cell, concerned with 

 penetration into an egg cell. 



Action potential. A rapid change in the membrane 

 potential, which signifies excitation in cells gen- 

 erally. 



Activation. The stimulation of an egg cell to start 

 development. 



Active transport. Any process by which a cell ex- 

 pends energy to foster the entrance or exit of a 

 substance through its boundary membrane. 



Adaptation. Fitness to live and reproduce. Any char- 

 acteristic of an organism fitting it to live and repro- 

 duce. 



Adhesion. Attraction between molecules of different 

 substances. 



Adrenal glands. A pair of ductless glands located 

 near the kidneys. 



Adrenalin. A hormone secreted by the adrenal 

 glands. 



Adsorption. The accumulation of a dissolved sub- 

 stance at the surface between two phases. 



Afferent fiber. A nerve fiber carrying impulses 

 from a receptor toward the central nervous system. 



Algae. Simple plants (thallophytes) containing 

 chlorophyll. 



Alleles. Genes occupying the same locus in homol- 

 ogous chromosomes, and which, therefore, segregate 

 from each other at reduction. 



Alternation of generations. The alternate succes- 

 sion of sexual and asexual forms, each producing 

 the other. 



A microscopic. Too small to be seen with either the 

 microscope or the ultramicroscope; i.e., less than 

 about 1 m/i in diameter. 



Amino acids. Organic acids possessing both the car- 

 boxyl radical (COOH) and the amino radical 

 (NH«) . 



Amitosis. Division of a cell without the appearance 

 of chromosomes. 



Amoeboid movement. Movement of a cell by means 

 of pseudopodia. 



Amylopsin. An enzyme, secreted by the pancreas, 

 which partially hydrolyzes starch. 



Anaerobic metabolism. Metabolism without the con- 

 sumption of free oxygen. 



Analogous organs. Organs having structural similar- 

 ities correlated with similarity of function and not 

 due to genetic relationship, (cf. Homologous or- 

 gans.) 



Anaphases. The period of mitosis during which 

 the daughter chromosomes move toward opposite 

 poles. 



Anther. The part of a stamen containing the pollen. 



Antheridium. Organ found in most embryophytes, 

 in which the sperm cells are formed. 



Anus. The terminal opening of the enteron. 



Aphids. Plant lice, small sucking insects parasitic on 

 plants. 



Apical cell. A cell at the apex of the growing point 

 of many thallophytes, bryophytes, and pteropsi- 

 dans, which by continued growth and cell divisions 

 produces all the tissues of the plant. 



Archfgontum. Organ found in most embryophytes, 

 in which the egg cells arc formed. 



Artery. A muscular, elastic blood vessel, carrying 

 blood from the heart to the capillaries, (cf. Vein.) 



Asexual reproduction. Any method of reproduction 

 not involving fertilization. 



Assimilation. The production of protoplasm from 

 food substances in the living cell. 



Aster. A radiate structure surrounding each end of 

 the spindle during cell division. 



Atoms. The elementary particles that enter into 

 chemical reactions. 



Auricle. A relatively thin-walled chamber of the 

 heart that receives blood from the veins and pumps 

 it into the vcnticle. 



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