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. 6 7 .-Cells" of yeast P hvlL The nucleus can be seen 
(Saccharomyces sp.}. Some of only after the cell is stained. 
the cells are budding. The ,.. , . . .. .... 
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 
