LESSONS IN BOTANY. 191 



If some of this scum be spread out on a piece of glass and exam- 

 ined with a pocket magnifier, fine grains may be seen scattered 

 through the mass. These grains, called torulse, are the active 

 part of the yeast. They are living bodies, as shown by the ra- 

 pidity with which they grow and multiply. Each torula is a cell 

 tvhich gives rise to minute buds, which grow rapidly and finally 

 become detached from the parent cell, though generally not till 

 they have themselves developed other buds. When yeast is dried 

 and burned it gives rise to an odor similar to that which arises 

 from burning meat. Dried yeast forms a fine powder which dif- 

 fuses itself rapidly through the air, so that a solution of sugar 

 open to the air, if kept warm, will soon begin to ferment from 

 yeast particles received from the air. 



Infuse or steep some hay in warm water for an hour or so, 

 then filter it and set the filtrate in a warm place, and from time 

 to time notice the changes which take place in it. At first the 

 solution is clear, after 24 or 36 hours it becomes turbid, as did 

 the yeast solution, and at length a scum forms on the surface 

 and the solution acquires a putrid odor. These changes are due 

 to the growth and multiplication of Bacteria. All forms of putre- 

 faction are due to fermentation set up by bacteria of different 

 kinds. A living thing dies, other things still living attack the 

 dead body and speedily convert it into gases and liquids which 

 may be used for food by the same or other forms of life. 



Bacteria may be developed in almost any vegetable infusion. 

 They are minute ; they cannot be studied in detail without pow- 

 erful instruments, but it is interesting to note the work of these 

 minute but numerous beings, which in some sense are the sub- 

 stratum of life. When dry the bacteria forms a dust of minute 

 grains which is diffused through the air everywhere, furnishing 

 the material for fermentation wherever the air can go freely. 

 They have size and form, for they may be filtered out of the air. 



Take three test tubes and fill each about one-third full of 

 freshly made hay infusion; cause the solutions in two of the 

 tubes to boil for a few minutes, then while it is boiling cork one 

 of the tubes with a mass of cotton wool and let it boil for a min- 



