THE YEAST PLANT 
321 
food), and those that live on dead plants or animals are 
called saprophytes (Greek, sapros, rotten; phyto, plant). 
274. The Yeast Plant. This plant is a unicellular fungus, 
too small to be seen by the naked eye. It is oval or almost 
round in shape, and nearly colorless. It has all the parts 
of a typical cell, although the nucleus cannot be seen without 
a special stain. Because it 
lives upon dead vegetable 
matter, it is a saprophyte. 
The Work of the Yeast 
Plant. — In the making 
of bread, we know that: 
(1) yeast secretes an en¬ 
zyme which breaks up 
sugar into simpler sub¬ 
stances; (2) in this pro¬ 
cess alcohol is formed and 
carbon dioxide is set free; 
(3) the yeast lives on the 
proteid substances in the 
flour; (4) both the gas 
which makes bread light 
and the alcohol are driven 
off by the heat of the oven 
when the bread is baked. 
Use is made of the en¬ 
zymes and yeast in the 
making of beer, ale, and porter. Before the action of bacteria 
and yeast were understood, much trouble was experienced in 
getting uniform products, owing to the presence of unde¬ 
sirable bacteria and yeasts. The possibility of making pure 
cultures, the use of the microscope, as well as the tests which 
are made in the laboratories at every step of the manufacture, 
have placed the industries of bread-making and brewing on 
a scientific basis. 
Figure 291 . — Fermentation Tubes. 
The bulb is filled with nutritive liquid 
containing yeast plants. As they grow 
they form carbon dioxide gas which col¬ 
lects in the bulb, forcing the liquid into 
the upright arm. These tubes are used 
in estimating the number of yeast plants 
in a substance and the rapidity of their 
growth. In water analysis, the forma¬ 
tion of gas with certain media indicates 
pollution. 
