12 MICROSCOPIC PLANTS. [CHAP. 



plant (Toruld), 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 cellulose, 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 Torula 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 

 FlG Ig 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, 



