Glossary - 689 



Fossils. Remains or traces of dead organisms pre- 

 served by natural processes. 



Fruit. A structure formed from the ovule case (and 

 sometimes other parts) of a flower, containing the 

 seed or seeds. 



Fuel substance. A substance that can be metabo- 

 lized to yield energy in the organism. 



Fuel value. The quantity of energy liberated by the 

 oxidation of a substance in the organisms; usually 

 expressed in Calories per gram. 



Fungi. Thallophytes without chlorophyll (often 

 used as including the bacteria) . 



Galvanotropism. Movement determined by the di- 

 rection of an external electric current. 



Gamete. A cell that unites with another cell in 

 sexual reproduction. 



Gametophyte. The sexual generation of a plant. 

 (cf. Sporophyte.) 



Ganglion. A group of nerve cells, especially the cell 

 bodies. 



Gastric juice. The fluid secreted by the gastric 

 glands. 



Gastrula. An embryo in the two-layered stage, con- 

 sisting of ectoderm and endoderm enclosing the 

 archenteron. 



Gastrulation. The formation of the gastrula from 

 the blastula. 



Gel. A mixture of solid or semisolid consistency con- 

 taining a large proportion of liquid entrapped in 

 the meshes of its solid component. 



Gelation. The change of a fluid dispersion into a gel 



Gene. A unit hereditary factor, probably a definitive 

 segment of a DNA molecule, capable of self-tem- 

 plated replication and mutation. 



Generator potential. A change of membrane po- 

 tential induced by stimulation of a receptor cell, 

 which determines the strength and frequency of its 

 excitational discharges. 



Genetic. Pertaining to internal factors determined 

 by the descent of an organism. 



Genetic code. Three svmboled system of base-pair 

 sequences in DNA; indirectly determines the 

 amino acid sequence in the enzymes and other 

 protein components synthesized by each organ- 

 ism. 



Genotype. The genetic constitution of an organism 

 or a group of genetically identical organisms, (cf. 

 Phenotype.) 



Genus. A taxonomic group, next above a species. 



Geological. Pertaining to the structure and history 

 of the earth. 



Geotropism. Movement determined by the direction 

 of gravitational force. 



Germ cells. The reproductive cells and all other 

 cells that stand in line of ascent of the reproductive 

 cells (cf. Somatic cells) . 



Germ plasm. The germ cells collectively. 



Germination. The beginning of growth from a spore 

 or a seed. 



Gill slits. A series of paired openings from the sides 

 of the pharynx through the body wall to the ex- 

 terior, found in all chordatcs, at least in embryonic 

 stages. 



Gills. External respiratory organs of aquatic ani- 

 mals. 



Gland. A cell or organ specialized for secretion. 



Glucose. C^H^O,,, the most common monosaccha- 

 ride, the chief fuel substance of most organisms. 



Glycerol. C,H s (OH)„ a water-soluble compound 

 entering into the synthesis of fats and phospholipids 

 and derived from them by hydrolysis; commonly 

 called glycerine. 



Glycogen. A complex carbohydrate (polysaccha- 

 ride) , one of the chief stored food substances of 

 most animals and fungi. 



Glycolysis. Anaerobic catabolism of glycogen, down 

 to the level of pyruvic acid (or, in some cases, 

 lactic acid) . 



Gonads. The essential reproductive organs of multi- 

 cellular animals, in which the gametes are formed. 



Gonia. The germ cells in the period of mitotic mul- 

 tiplication preceding maturation. 



Gonoducts. The ducts through which the gametes 

 reach the exterior of the body, found in most ani- 

 mals above the coelenterates. 



Grana. Small rounded bodies inside chloroplasts, as 

 seen in electronmicrographs. 



Growing point. The region at the apex of each fila- 

 ment, stem, or root, to which growth and cell divi- 

 sion are largely confined in most plants. 



Growth period. The period in the development of 

 the germ cells preceding the meiotic divisions, dur- 

 ing which they increase greatly in size and dur- 

 ing which synapsis occurs and the tetrads are 

 formed. 



Growth stage. The intermitotic (nondividing) stage 

 of the cell; often called the resting stage. 



Guard cells. Cells surrounding each stoma, which 

 open and close the stoma. 



Habit. A usually invariable response determined by 

 the previous experience of the individual; a condi- 

 tioned reflex. 



Habitat. The particular kind of environment in- 

 habited by a particular kind of organism. 



Haploid. The number of chromosomes in each 

 gamete; i.e., a single "set" of chromosomes, (cf. 

 Diploid.) 



Hemoglobin. An iron-containing protein in the 

 blood of many animals, in combination with which 

 oxygen is transported through the body. 



Hepatic portal system. A part of the circulatory 

 system, in all vertebrates, by which blood is carried 

 from the stomach and intestine to the liver; starts 

 and ends in a capillary system. 



Heredity. The transmission of genes from ancestors 

 to descendants through the germ cells. 



Hermaphrodite. A monoecious animal, bearing both 

 testes and ovaries. 



Heterotrophic. Requiring organic compounds among 

 the food substances taken into the organism (cf. 

 Autotrophic.) 



Heterozygous. Having the two genes of an allelic 

 pair different, (cf. Homozygous.) 



Holophytic. Rind of nutrition found in green 

 plants, which utilize light energy for synthesis of 

 organic matter, 



