864 Appendix 



Y chromosome, a special chromosome associated with the sex of the organism. In 

 human beings this chromosome is present only in males. 



Yeast (A.S. gist, ferment), unicellular, chlorophyll-less plants capable of fermen- 

 tation; some are pathogenic. 



Yolk (yok) {yolke, yellow), stored food in the egg cytoplasm. 



Zone of tissue difFerentiation, the point in young plant roots and stems where 

 adult tissues are being formed. 



Zoogeography (zo o je -og' ra fi) (Gr. zoon, animal; ge, earth; graphein, to write) ^ 

 geographic distribution of animals in space. 



Zoology (zo-ol'oji) (Gr. zoon, animal; logos, study), study of animals. 



Zoosporangia (zo o spor -an' ji a) (Gr. zoon, animal; sporos, spore; anggeion, ves- 

 sel), a structure in which motile zoospores develop. 



Zoospore (zo'ospor), motile spore. 



Zygospore (zy' go spor) (Gr. zygotos, united), spore formed by the union of two 

 gametes (male and female sex cells). 



Zygote (zy' got) (Gr. zygotos, united), fertilized egg cell after fusion with male 

 gamete. 



Zymase (zy'mas) (Gr. zym, leaven; ase, enzyme or ferment), enzyme (in presence 

 of oxygen) which converts glucose and other carbohydrates into carbon 

 dioxide and water or (in absence of oxygen) into alcohol and carbon 

 dioxide or into lactic acid. 



Zymogen (zy'mo jen) (Gr. zym, ferment; gen, to form), forerunner of an enzyme 

 (pre-enzyme) ; a substance which is produced and later becomes an 

 enzyme when it is activated by another substance (probably another 

 enzyme). 



Zymosis (zy-mo'sis), any form of fermentation, especially morbific (L. morbus, 

 disease; facto, to make). 



