262 Introduction to Botany. 



by green plants. They are rounded or ellipsoidal (Fig. 

 133, a), and are about .0015 to .015 millimeter in diameter. 

 Each individual consists of a single protoplast, surrounded 

 by a delicate wall. Investigation into the life history of 

 yeasts seems to have shown that they are really spore-like 

 forms which have been produced by certain species of 

 filamentous Fungi, but however this may be, the yeasts 

 are capable of sustaining an independent existence, and 

 of multiplying both by budding and by internal spores. 



159. Reproduction of Yeasts. — In budding, a knob-like 

 outgrowth is produced which finally becomes separated and 

 grows to the size of the individual from which it sprang. 

 In internal spore-formation, the cell protoplasm breaks up 

 into several rounded bodies that escape and finally germi- 

 nate, producing forms like the one from which they sprang. 

 The internal spores are apt to be formed when the food is 

 running short ; and they are evidently useful in tiding over 

 unfavorable conditions. 



160. Yeasts in Bread-making. — Yeasts obtain part of 

 their food from weak solutions of sugar, and in so doing 

 convert the sugar into alcohol and carbonic acid ; their 

 usefulness in the raising of bread is due to this action. 

 When the sponge is made, the ferment known as diastase, 

 which was produced in the grain of wheat for the purpose 

 of digesting the starch when the seed germinates, carries 

 on the process of starch digestion in the sponge, changing 

 a part of the starch into grape sugar. Then the yeast 

 plants begin their action on the sugar, and the carbon di- 

 oxide produced along with the alcohol becomes entangled 

 in the sticky mass and causes it to puff up. If, now, while 

 the gas is still forming, the flour is worked in with the 

 sponge, the gas is produced at all points throughout the 

 mass of dough and raises it. The alcohol which is produced 



