706 



INDEX 



Wood, of dicotyledonous stem, 166, 167; 



uses of, 577-578 

 Woodpecker, downy, 557 

 Work, relation to diet, 335 

 World, how man has become conqueror, 



420-451 

 Worms, 228-231 

 Wren, house, 554 



Xerophthalmia, 



334 

 Xylem, 167 



deficiency disease, 333, 



Yeast, 190-193 ; and bread making, 191- 

 192 ; commercial, 191 ; conditions 

 favorable for growth of, 190 ; impor- 

 tance of, 192-193 ; life history of, 191 



Yellow fever, discovery of cause, 487- 

 488 ; how spread, 487 



Yucca, 93 ; pollination of, 94 



Zones, life, 307-308 

 Zygospore, 615 ; of Spirogyra, 202 

 Zygote, how formed, 616 

 Zymase, an enzyme, 191 



