gullet: food pipe or oesophagus iti many-celled 

 animals; in protozoa the food tube leading 

 into the cell. 



gymnosperms (jim'no-sperms) : plants of the 

 spermatophyte group that have uncovered or 

 "naked" seeds, such as the cone bearers. 



habit: behavior that has been learned so well that 

 it has become automatic. 



haploid number: the half number of chromo- 

 somes; the number found in each sex cell as 

 distinguished from the diploid or double 

 number found in all other cells. 



hemoglobin (he'mo-globe'in) : protein containing 

 iron in the red blood cells of vertebrates; it 

 carries oxygen by combining loosely with it; 

 the new substance oxyhemoglobin is bright 

 red. 



hemophilia (he-mo-fiire-a) : condition in which 

 the reduced ability of the blood to clot may 

 result in dangerous bleeding. 



herb (erb) : seed plant with a stem above ground 

 which does not become woody. 



heredity: passing on genes and thus charac- 

 ters from parents to offspring; the word is 

 used for the resemblance as well as for the 

 process. 



hibernation (high-ber-nay'shun) : a resting or 

 "sleeping" stage occurring in some animals 

 during the winter. 



hilum (high'lum) : scar left on seeds by the 

 breaking off of the stalk which attached the 

 seed to the fruit. 



Homo sapiens (hoh'mo say'pee-enz) : single spe- 

 cies within the genus Homo which includes 

 all mankind and some prehistoric forms. 



hormone: substance secreted by the cells of an 

 animal directly into the blood and carried to 

 other parts of the body where it regulates 

 and modifies activities. In plants the growth 

 substances are often called hormones. 



horsetails: perennial, herbaceous pteridophytes 

 with hollow, jointed stems; also called scour- 

 ing rushes. 



hybrid (high'brid) : the opposite of pure in ge- 

 netics. In a hybrid the two genes of a pair 

 are different. 



hybrid vigor: the increased hardiness and size 

 which sometimes results from outbreeding or 

 crossbreeding. 



hybridization (high'brid-iz-ay'shun) : the cross- 

 ing of individuals having contrasting char- 

 acters. 



hydrotropism (hy-dro'tro-pism) : tropism in which 

 the stimulus is moisture or water. 



hypha (high'fa) : a thread-like part of a mold. 



hypocotyl (high-po-cot'il) : that part of the plant 

 embryo or seedling which lies between the 

 point where the cotyledons are attached and 

 the upper part of the imdeveloped root. 



identical twins: twins developed from a single 

 fertilized egg, as distinguished from fraternal 

 twins. 



igneous (ig'nee-us) rock: very hard rock formed 

 from the cooling of molten rock. 



immunity: not being susceptible to a particular 

 disease. Natural immunity: immunity which 

 one has without treatment or without having 

 had the disease. Acquired immunity: immu- 

 nity which one gets either through recovery 

 from the disease or by appropriate inocula- 

 tion. Active immunity: immunity acquired 

 after introduction of bacteria or their prod- 

 ucts. Passive immunity: immunity acquired by 

 inoculation of antibodies made by some other 

 animal. 



impulse: as applied to a nerve, is that which is 

 carried along the axon either into or out of 

 the cell body of the nerve cell. 



inbreeding: breeding of close relatives; in plants 

 usually by self-fertilization. 



incisor (in-size'er) : in mammals the front teeth 

 in both jaws used for cutting; particularly 

 well developed in rodents. 



incomplete dominance: another name for blend- 

 ing inheritance. 



incubation (in-kew-bay'shun) : keeping eggs, em- 

 bryos, or young colonies of cells at an even, 

 favorable temperature during development ; 

 eggs of wild birds are incubated by the" 

 adults; eggs of domestic fowl are usually in- 

 cubated by artificial heat. 



independent assortment: Mendelian law of he- 

 redity which states that genes lying in dif- 

 ferent pairs of chromosomes are segregated 

 independently of one another; also law of 

 unit characters. 



infectious (in-fek'shus) disease: any disease which 

 may be caused by the entrance of a micro- 

 organism into the body; frequently transmit- 

 ted from one person to another, or conta- 

 gious. 



inoculation (in-oc'you-lay'shun) : introduction of 

 bacteria or viruses into the body or into 

 media suited to their growth; also introduc- 

 tion of antibodies or immune serum into the 

 body. 



insects: members of a very large class of arthro- 

 pods having three distinct body regions, 

 three pairs of legs, and often wings. 



instinctive behavior: complicated inherited be- 

 havior in which there is a series of reflexes, 

 each reflex serving as a stimulus to the next, 

 as web spinning by spiders or nest building 

 by birds. 



insulin: hormone secreted by the "islands of 

 Langerhans" in the pancreas; regulates sugar 

 metabolism. 



intelligence quotient or I. Q.: number represent- 

 ing the score obtained on an intelligence test 

 compared with the scores made by large num- 

 bers of people of the same age. 



invertebrates: animals that have no backbone or 

 rodlike structure (notochord) in the place of 

 the backbone. 



590 



