12 MICROSCOPIC PLANTS. [CHAP. 
plant (Zorula), which plays such an important part 
in the manufacture of beer. 
Each Yeast-plant consists, like Protococcus, of a 
single cell containing protoplasm, which has a clear 
space (vacuole) in it. The cell-wall consists of a sub- 
stance called ce//ulose, of which all wood is composed; 
it has no vitality, and therefore cannot grow, but is 
increased by fresh deposits formed by the protoplasm. 
On being chemically analysed, it is found to con- 
sist of carbon, hydrogen, oxygen, nitrogen, sulphur, 
and small quantities of other elements, which are 
arranged in the form of protein, fat, and cellulose, 
much the same as in protococcus. In the latter 
plant we saw that it had the power of building up 
these substances out of such simple compounds as 
carbonic anhydride, ammonium nitrate, and a few 
mineral salts, all of which are found in rain-water. 
But if we place Zoru/a in rain-water, it cannot grow 
at all. To enable it to grow, we must place it in 
a fluid containing sugar, ammonium tartrate, and 
mineral salts. If we place this solution, with a little 
yeast, in a bottle, and keep 
it corked up for a day or 
two, the cork will probably be 
driven out with great force 
and a mighty effervescence 
will take place in the fluid, 
If we now fix a glass tube 
through the cork, and se- 
curely fasten an empty blad- 
der to the upper end, we shall see that it will 
gradually become filled with gas. If we are chemists, 
