232 PRACTICAL BOTANY 



rarely formed. It reproduces itself very .abundantly by means 

 of conidia. Plants branch profusely at their ends, and from 

 the tips of these branches conidia are formed (Fig. 187). The 

 number of these conidia is often so large that when the sub- 

 stance supporting the plants is slightly shaken a small cloud 

 of spores arises. 



225. Yeasts. The yeasts (Saccharomycetes) constitute a group 

 of plants of somewhat doubtful classification. Since occasion- 

 ally they form an ascus-like sac in which spores are formed, 



they are often classed with 

 the ascomycetes. They are 

 extremely simple, and are 

 more interesting because of 

 their manner of life than 

 because of their structure. 



A yeast plant is a single cell 

 FIG. 188. Yeast plants (Saccharomycetes) _ 



(Fig. 188). It usually repro- 



a, a plant from which a bud has begun 2 . ' , , ,11 



to grow; b and c, plants with two buds, duces itself by a method 



Note the vacuoles in the plants. Greatly o f vegetative reproduction 



known as budding. The 



buds, before becoming separated from the parent cells, may 

 bud again- and again until a chain of plants is formed. If a 

 cake of commercial yeast is examined, it is found, in addition to 

 the large starch grains nearly always occurring in yeast cakes, 

 to consist of thousands of yeast cells, some single and some in 

 process of budding. If a cake of yeast is kept at room tempera- 

 ture, 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 break down the sugar, thus forming from it 

 small quantities of alcohol and carbon dioxide. The carbon- 

 dioxide gas forms the "air spaces" in the dough, which cause 

 the phenomena known as " rising." 1 In cooking the dough 



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

 yeasts certain bacteria produce a putrefactive fermentation within the dough. 



