HOW PLANTS BENEFIT AND HARM MANKIND 171 



Hundreds of millions' yearly damage may be laid directly to them. 

 More than that, they are doubtless responsible for one half of the 

 total human deaths. This is because of their parasitic habits. 



Yeast. Although as a group the fungi are harmful to man in the 

 economic sense, nevertheless there are some fungi that stand in a decidedly 

 helpful relationship to the human race. Chief 

 of these are the yeast plants. Yeasts are 

 found to exist in a wild state in very many 

 parts of the world. They are found on the 

 skins of fruits, in the soil of vineyards and 

 orchards, in cider, beer, and other fluids, 

 while they may exist in a dry state almost 

 anywhere in the air around us. In a culti- 

 vated state we find them doing our work as A, yeast plant bud just form- 

 the agents which cause the rising of bread, ing ; B, bud almost ready 

 and the fermentation in beer and other alco- to leave parent cell. Note 

 holic fluids. thc nucleus (N) dividing 



Size and Shape, Manner of Growth, etc. (After Sedg- 



m , , , . wick and VV ilson.) 



The common compressed yeast cake contains 



millions of these tiny plants. In its simplest form a yeast plant is a 

 single cell. If you shake up a bit of a compressed yeast cake in a mixture 

 of sugar and water and then examine a drop of the milky fluid after it has 

 stood overnight, it will be seen to contain vast numbers of yeast plants. 

 The shape of such a plant is ovoid, each cell showing under the micro- 

 scope the granular appearance of the protoplasm of which it is formed. 

 Look for tiny clear areas in the cells ; these are vacuoles, or spaces filled 

 with fluid. The nucleus is hard to find in an unstained yeast cell; it 

 can, however, be found in specimens which have been prepared by stain- 

 ing the previously killed cells with iron-hsematoxylin. 1 Yeast cells repro- 

 duce very rapidly by a process of budding, a part of the parent cell 

 forming one or more smaller daughter cells which eventually become free 

 from the parent. 



Most yeast plants seem to produce spores at some time during their 

 existence. The spores are formed within a yeast cell, as many as four 

 being produced within a single cell. These spores, under proper condi- 

 tions, will germinate and give rise to new plants. 



Conditions favorable to Growth of Yeast. Under certain conditions 

 yeast, when added to dough, will cause it to rise. We also know that yeast 

 has something to do with the process we call fermentation. The following 

 home experiment will throw some light on these points : 



Label three pint fruit jars A, B, and C. Add one fourth of a com- 

 pressed yeast cake to two cups of water containing two tablespoonfuls of 

 molasses or sugar. Stir the mixture well and divide it into three equal 

 i See Lee, Vade Mecum, or Sedgwick and Wilson, General Biology. 



