98 THE VEGETATIVE FUNCTIONS OF PLANTS 



that the last two are also the results of another important 

 life-process of plants. 



98. What is Yeast? — For centuries men employed yeast 

 in baking and brewing without having the remotest idea 

 as to what it really is, or of how it causes fermentation. 

 This was because they did not inquire into the matter. 

 It was not until 1680 that Leeuwenhoek, a Dutch natu- 

 ralist, discovered that liquid yeast always contains tiny 

 floating globules. It was 150 years after this that a 

 French scientist, Cagniard de la Tour, discovered that 

 yeast is a living organism, and soon thereafter another 

 observer, Turpin, demonstrated that yeast is really a 

 plant, closely related to the fungi. Thanks to the 

 painstaking work of many other students, and especially 

 of Pasteur, we now have a detailed knowledge of the 

 structure and activity of the yeast plant. It is related 

 to the sac-fungi (Ascomycetes) . 



99. Microscopic Appearance. — If a drop of yeast mix- 

 ture that has been fermenting vigorously is examined 



under the microscope, the indi- 

 vidual yeast plants may be 

 readily observed (Fig. 67). 

 They are seen to be unicellular 

 plants, globular or ellipsoidal in 

 form, of various sizes according 

 to age, and devoid of chloro- 



FiG. 67.-Cells' of yeast P^l- The nucleus can be seen 

 {Saccharoynyces sp.). Some of only after the cell is stained. 



the cells are budding. The ^^ c ,1 a n mi i 



clear spaces are vacuoles. Many of the larger cells Will be 



seen to have small outgrowths 



or buds, also of various sizes according to age. It is by the 



formation of the buds, that is, by budding, that the plant 



