GLOSSARY-INDEX 425 



worker bees. Female bees whose sexual organs do not mature, and who 



perform the work of the colony, 

 xerophytes (Gr. xeros dry + phyton a plant). Plants inhabiting 



very dry regions, 

 xylem (Gr. xylon = wood). The layers of hard woody cells inside the 



cambium layer in exogenous stems; the wood, 105. 

 yeast, 78, 235. 

 yolk. The food material deposited in an egg for the nourishment of the 



developing embryo, 250. 

 zooid (Gr. zoon = an animal). A more or less independent member of a 



compound organism, like a hydroid, 148. 

 zoospore (Gr. zoon = an animal + spora) . Spores which swim by the 



agency of motile flagella or cilia, 93, 136. 

 Zoothamnium, 92. 



zygapophyses. Articular processes in vertebrae, 177. 

 zygospore (Gr. zygon = a yoke + spora). A spore formed by the union 



of two gametes, 94, 262. 

 zygote (Gr. zygon = a yoke). A cell formed from the union of two others 



in sexual reproduction; a zygospore, 94, 262. 

 zymogenic. Giving rise to fermentations. 



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