246 INTRODUCTION TO BOTANY 



is a sac fungus which has almost lost the habit of reproduction 

 by means of ascospores, the sac being rarely formed. It repro- 

 duces itself very abundantly by means of conidia (fig. 191). 

 The number of these conidia is often so large that when the 

 substance supporting the plants is slightly shaken, a small 

 cloud of spores arises. 



234. Yeasts. The yeasts constitute a group of plants of 

 somewhat doubtful classification. Since they occasionally form 

 sacs in which spores are formed, they are often classed with 

 the sac fungi. They are extremely simple, and are more inter- 

 esting because of their manner of life than because of their 



structure. A yeast plant is 

 a single cell (fig. 192). It 

 usually reproduces itself by 

 a method of vegetative repro- 

 duction known as budding. 

 The buds, before becoming 

 FIG. 192. Yeast plants separated from the parent 



a, a plant from which a bud has begun cells, may bud again and 

 to grow; b and c, plants with two again until a chain of plants 



buds. Note the vacuoles in the plants. . . j T r i r 



Greatly enlarged J formed. If a cake of com- 



mercial yeast is examined, it 



is found, in addition to the large starch grains nearly always 

 occurring in yeast cakes, to consist of hundreds of thousands 

 of yeast cells, some single and some in process of budding. 

 If a cake of yeast is kept at room temperature, the plants 

 soon continue their growth, and other organisms (bacteria and 

 molds) also grow, so that the yeast " spoils." 



When yeast plants are placed in dough, they grow with 

 great rapidity. They live upon the solutions in the dough, 

 and in so doing they break down the sugar, thus forming from 

 it small quantities of alcohol and carbon dioxide. The carbon- 

 dioxide gas forms air spaces which cause the phenomenon 

 known as rising. 1 During the process of baking, the air spaces 



1 Salt-rising bread owes its peculiar quality to the fact that, instead of 

 yeasts, certain bacteria produce fermentation within the dough. 



