Microorganisms in Relation to Man 565 



Exercise. Examine now the liquid that was heated and kept 

 unexposed to the air. Do you find similar bodies in it? The boiling 

 would destroy any living organisms in the fresh fruit juice , and the 

 stopper would prevent others from entering. 



What do you find in the other liquid which was heated and left 

 exposed to the air ? Are the yeast plants evident in any way ? From 

 what source can they come? Fermentation in this tube indicates 

 that the yeast plants entered it from the air. The simple mixture of 

 flour, salt, and water developed fermentation which seems to have 

 been due to the work of yeast plants. In this case the yeast plants 

 may have come from the air or may have been in the flour and water. 



Distribution of yeast. How difficult it is to keep the micro- 

 scopic plants out of fruit and other preparations may be in- 

 ferred from a few results of investigation. Samples of apple 

 ciders made in the usual way showed from 50,000 to 30,000,000 

 yeast plants or spores per cubic centimeter. A cubic centi- 

 meter of apple butter contained nearly 10,000,000, and an equal 

 quantity of currant jam had the moderate number of 40,000,000. 

 Fortunately most varieties of the yeast plant are harmless; 

 but a few cause disease or serious intestinal troubles. 



The facts indicate that yeast is widely and abundantly dis- 

 tributed. How is this done? Do the yeast plants themselves 

 get carried about or do they, like molds, produce " seed " or 

 spores which are transported by the moving air? If it were 

 practicable to carry on a series of experiments with the yeast 

 plants, putting them into different kinds of conditions, it 

 would be found that yeast plants in certain unfavorable con- 

 ditions, as when deprived of food, do produce spores. The 

 little plants divide into two or more minute bodies which 

 eventually break through the cell wall of the yeast and escape 

 as spores. Spores are exceedingly light and small, and are 

 carried like impalpable dust far and wide in a gently moving 

 air. The spores successfully resist unusual extremes of heat, 

 cold, and dryness. They are found practically everywhere, 

 as every case of fermentation of any sort indicates. They are 

 in the air, in water, in the soil, and are attached to all kinds of 



