the Yeast Plant . 
63 
we find, in the course of one to three hours, that some of the cells come 
to the surface and that, although glycogen is still present in them, a marked 
decrease in the amount has taken place. In the cells which remain at the 
bottom glycogen is still abundant, but in these also a considerable decrease 
in amount is observable. After four or five hours most of the cells become 
distributed throughout the fluid and fermentation is more active. 
During the next 5-15 hours the glycogen practically disappears in all 
the healthy cells, and, as we have already noted, this is the period of 
greatest vegetative activity. Between 10-20 hours the glycogen may 
reappear in small quantities. At first the increase is very slow and for 
some time almost imperceptible. It then increases rapidly, and in two 
hours a considerable amount may appear, but not enough to interfere with 
the nuclear vacuole. It is significant that this sudden increase is coincident 
with the decrease in budding activity. 
From 20-30 hours the glycogen continues to increase but slowly; the 
cells are still budding but less vigorously; fermentation is increasing, and 
the nuclear vacuole during this period is very large. Immediately after or 
near the height of fermentation the glycogen increases rapidly, and this 
coincides with the decrease of the nuclear vacuole. 
All the cells now contain a large quantity of glycogen, and a number 
of cells are found which apparently contain little else. So far very few 
cells have settled to the bottom, but about this time a large number of 
cells sink. 
On examining these we find that practically every cell is filled with 
glycogen. Fermentation may be carried on for some time by the yeast 
which remains at the top, the cells gradually sinking as they become filled 
with glycogen, but fermentation is finished for the time being when they 
sink to the bottom. 
If the fermenting fluid is not exhausted, after some time air-bubbles 
begin to rise from the bottom, and in the course of 5-10 hours a large 
number of the cells again come to the surface. This is attended by the 
disappearance of the greater part of their glycogen. They attain a maximum 
and again sink to the bottom, containing a large amount of glycogen. 
This process may be repeated two or three times, but each time as 
the fluid becomes less concentrated the height of fermentation is reached 
sooner, and the cells sink with much less glycogen than in the first fermenta¬ 
tion. This explains an apparent periodicity in fermentative activity which 
occurs when yeast is grown in a relatively large quantity of Pasteurs 
solution. 
A slight foam may remain at the surface for a considerable time after 
the complete cessation of activity. Such a foam invariably consists of 
chains of cells which entangle the air-bubbles, and are thus enabled to 
remain at the surface although they may contain a large amount of glycogen. 
