38 TEXTBOOK OF BOTANY 



clear. There is some reason to think that they may be descendants 

 of more complex fungi which have degenerated in the course of evolu- 

 tion ; but this is by no means certain. 



58. Historical Note. In 1680, Leeuwenhoek noticed that yeast 

 seen under the microscope consists of small, rounded particles. About 

 1838 it was discovered by several observers that the particles which 

 Leeuwenhoek had seen are really plant cells, and it was concluded 

 that these living cells in some way produce the chemical changes that 

 go on only when they are present. Nearly twenty years later, Pasteur 

 took up the study of yeasts at the request of German brewers, who 

 were greatly troubled by impure yeasts. In 1856, Pasteur found a 

 method of securing " pure cultures " that is, cultures which con- 

 tain only one variety of yeast and no other organism whatever. By 

 this method he studied many different forms of yeast ; he not only 

 succeeded in revolutionizing the brewing industry, but his method of 

 pure cultures was the starting-point of the modern study of bacteria 

 as well. 



