GLOSSARY 449 



Heliotropism. Response to sunlight. 



Hermaphrodite. An individual equipped with the reproductive organs 

 of both sexes. 



Heterogamy. The character of producing germ cells that may be dis- 

 tinguished as eggs and sperms. 



Heterospory. The character in flowering plants of producing two dif- 

 ferent types of spores by the sporophyte generation, one type to 

 develop into the male gametophyte, the other into the female game- 

 tophyte. 



Heterozygous. An individual is said to be heterozygous when it carries 

 in its genetic make-up the possibility of transmitting either of two 

 alternative characters. 



Hibernation. The winter f)eriod of quiescence characteristic of the life 

 cycles of many animals. 



Histology. The science that deals with the cellular structure of tissue. 



HoLOPHYTic nutrition. The essentially constructive metabolism of green 

 plants involving the synthesis of carbohydrates and of proteins from 

 simple inorganic substances. 



HoLozooic nutrition. The essentially destructive metabolism of ani- 

 mals whereby they utilize the carbohydrates derived from plant 

 metabolism as their source of energy and plant proteins, or proteins 

 derived indirectly from plants, in constructing their own proteins. 



Homoiothermal (Adjective). An animal whose body temperature is 

 indef)endent of that of the environment. 



Homologous. Structures are said to be homologous if they are derived 

 from the same embryological origins. 



Homologous chromosomes. Chromosomes bearing the same or alter- 

 native characters. 



Homozygous. An individual is said to be homozygous when its genetic 

 composition carries only one type of gene for any given character. 



Hormone. A chemical agent in the circulatory fluid, particularly in ver- 

 tebrates, that influences the metabolism of the body in a particular 

 way. Hormones diffuse readily through membranes and are not 

 easily destroyed by heat. In general they are produced by special 

 glands of internal secretion. 



Human biology. The principles of Biology that are concerned with 

 human life. 



Hydranth. An organ that is the terminal of a branch in the Hydrozoa. 

 Hydranths are of two sorts, feeding and reproductive. 



