Bacteria and Fungi 



261 



however, and get their food either from organic matter or 

 from Hving plants. The plant body is always made up of 

 filaments like those of the algcX\ In the higher forms, hke the 

 mushrooms, the filaments are massed together into a com- 

 pact, solid structure. 



The yeasts and molds, and most of the mushrooms, are 

 saprophytes ; the smuts, rusts, and some of the. mildews are 

 parasites upon the higher plants, and they cause serious losses 

 to the farmer and gardener. A few of the mushrooms are edible 

 and furnish small quantities of food for animals and man. 



The yeasts. In the making of bread, yeasts are of primary 

 importance. They are small, one-celled plants that multi- 

 ply very rapidly, and when properly mixed with flour and 

 water they develop in all parts of the dough. The yeasts 

 have within them enzymes which change part of the sugar 

 that is present into carbon dioxid and alcohol. The carbon 

 dioxid accumulates in bubbles and makes the dough light. 

 When the dough is put into a hot oven the alcohol is vaporized, 

 and together with the carbon dioxid it is driven oft* into the 

 air. The high temperature kills the yeast, bakes the dough, 

 and changes some of the starch 

 into its soluble form, dextrin, 

 which makes it more readily 

 digestible. Sour bread is pro- 

 duced when the yeast that is 

 added contains acid-foniiing 

 bacteria which change part of 

 the alcohol into acetic acid. 



Yeasts and bacteria are the 



Fig. 15,3. Yeast cells and chains of cells. 

 Above arc three cells, each containing four 



juice into cider and vinegar. rcsUng spores. 



organisms that change fruit 



