546 TEXTBOOK OF BOTANY 



wrapped in waxed paper while still warm from the oven, mold spores 

 are more likely to be kept out, and the bread keeps in good condition 

 for a much longer period. 



Sexual reproduction can be observed between mycelia of bread mold 

 started from different spores on the same culture plate. Where hyphae of 

 the two mycelia come in contact, the adjoining walls dissolve and the 

 contents of the two "cells" fuse. The union of the two protoplasts, or 

 gametes, results in a heavy-walled black zygote. Upon gemiination of a 

 zygote, a short hypha develops which terminates in a globular spor- 

 angium containing spores. From these spores branching hyphae grow, 

 and the development of a new mvcelium begins. 



Fig. 237. Yeast (Saccharomyces) : cells and branching filaments. Above are three 

 cells, each containing four resting spores. Courtesy of World Book Co. 



Yeasts. In the discussion of respiration it was noted that the fomia- 

 tion of alcohol and carbon dioxide from a sugar solution may be brought 

 about in oxygen-free containers if certain yeasts are present. When 

 yeasts are thoroughlv mixed with flour and water, their activities bring 

 about the "rising" of bread dough. The carbon dioxide resulting from 

 the oxidation of sugar accumulates in bubbles throughout the dough and 

 makes it porous. While the bread is baking, the alcohol vaporizes and 

 with the carbon dioxide passes from the bread into the air. 



The yeasts have little resemblance to the bread mold described above, 

 for they are generallv microscopic one-celled plants more or less ellip- 

 soidal in form. Some species of yeast are so small that thev resemble the 

 largest bacteria. Although a single yeast cell is microscopic, the enomious 

 numbers that develop in a sugar solution soon make it cloudy, and 

 eventuallv sediment collects at the bottom of the vessel. The yeast cakes 

 sold bv commercial firms consist of starch grains and yeast cells pressed 

 together into a compact mass. 



