178 ELEMENTS OF PHYSIOLOGY 



height; this is called mold (Fig. 152). It grows, for 

 instance, upon moist bread in warm weather. Mold forms 

 a kind of network as it grows through any substance that 

 N will nourish it, and sends up stalks with 

 knoblike ends, the knobs being full of 

 minute spores. 



305. Yeast Think, if you can, of 

 small plants only about ^ for of an incn 

 in length, composed usually of one oval 

 cell, a plant that makes new plants by 

 "budding," or the forming of a smaller 

 FIG. 152. Ring- cel1 on tne olc * one. Yeast cakes consist 

 worm Fungus in a of yeast plants scattered among the grains 



wiif Fi g . ( ^ mpare of flour of which the cake was made - 



Yeast plants multiply rapidly, doubling in 

 number in two hours (Fig. 153). They grow upon sugar, 

 decomposing it into alcohol and carbon dioxid. This pro- 

 cess is called fermentation. In wine making, alcohol is the 

 product sought; in bread making, carbon dioxid 

 is the useful product, this gas giving the bread 

 its lightness, while the small amount of alco- 

 hol formed is driven out by the heat in 

 cooking. 



306. Bacteria. But suppose the micro- 

 scope shows a much smaller vegetable organ- FIG. 153. The 



.. , r L Yeast Plant - 



ism which averages ^oWoo" f an mc ^ m 



diameter and is a one-celled vegetable that multiplies by 

 division like the ameba and other one-celled animals, and 

 lives upon albuminous substances only for its food. This 

 is a bacterium. Bacteria are called also microbes or 

 germs ; and they are the smallest of living organisms. A 

 bacterium that is rod-shaped is called a bacillus (Fig. 

 154). Under favorable conditions abundance of food 

 and considerable warmth and moisture bacteria may 

 double in numbers every half hour. Thus millions may 



