450 ELEMENTS OF BIOLOGY 



Hydrolysis. A chemical reaction in which water is one of the reacting 

 substances. 



Hydrostatic pressure. The pressure exhibited by a column of liquid 

 because of gravity. 



Hydrotropism. Response to water. 



Hypertonic. A solution containing such a concentration of solutes that 

 its osmotic pressure is greater than that of its surroundings. 



Hypostome. The structure in the feeding hydranth of Hydrozoa in 

 which the mouth is located. 



Hypotonic. A solution that because of its salt content has an osmotic 

 pressure less than that of its surroundings. 



Immunity. A state of the body that does not permit disease to develop 

 as a result of bacterial invasions. 



Inbreeding. Mating of individuals that are closely related. 



Incomplete metamorphosis. Describes insects which pass through three 

 stages in their life history — egg, nymph, and adult. 



Individual adaptations. Adaptive characters that are peculiar to an in- 

 dividual animal. 



Instinct. Responses that are reflex, automatic, independent of experi- 

 ence, and regular in occurrence. 



Insulin. A preparation derived from the islet tissue of the pancreas that 

 has to do with the regulation of the sugar content of the blood. 



Intelligence. Response that is variable, associated with experience, and 

 controlled by imagination. 



Internal fertilization. Fertilization occurring in the female body. 



Interstitial cells. Usually refers to the endocrine tissue in the ovary 

 and testis that is the site of production of a hormone which in- 

 fluences the development of the secondary sex characters and per- 

 haps the primary as well. 



Intra-cellular digestion. The digestion of foods within the cell. 



Ion. An atom or a combination of atoms that when dissolved bears 

 electrical charges. 



Irritability. The property of responding to some influence arising out- 

 side the cell. 



Isogamy. Some plants produce germ cells that cannot be distinguished as 

 eggs and sperm; they are known as isogamous plants. 



Isolecithal (Adjective). Eggs with a relatively small quantity of yolk 

 evenly distributed throughout the cell. 



