THE FOOD OF PLANTS. 97 



treated with a little yeast, one solution being kept cool (at 10- 

 15 C.), the other warm (at 25-30 C.), it will be found that in 

 the former only slight growth and multiplication of the yeast 

 takes place, while the second at once actively ferments, rapid 

 multiplication of the yeast at the same time setting in. 



We can easily ascertain by similar experiments that fermenta- 

 tion and propagation of the yeast takes place in sugar-containing 

 solutions, in darkness as well as in light (see further Section III.). 



1 See A. Mayer, Lehrbuch d. Gahrvngschemie, 1874, p. 107, and Untersuchun- 

 gen liber die alkoholiscJie Gcilirung, 1869. 



38. Bacteria. 



Bacteria are very widely distributed in nature. They are trans- 

 ported in all directions with the dust by currents of air, and under 

 favourable conditions display their vitality. We fill a number of 

 Erlenmeyer's flasks with a filtered fluid containing in solution in 

 every 100 parts of water, 1 part of grape sugar and 0'5 part of 

 meat extract. On the walls of the flask, and especially in the 

 meat extract, are numerous bacterium germs, and the solutions, if 

 allowed to stand for a few days, become very turbid, owing to 

 copious development of bacteria. By previous sterilisation, how- 

 ever, and arrangements for keeping the fluids sterile, this develop- 

 ment of bacteria can be prevented. 



The Erlenmeyer's flasks are first rinsed with concentrated Sul- 

 phuric acid, then with boiled distilled water. After 100-200 c.c. 

 of the above solution have been introduced, the flasks are tightly 

 plugged with cotton wool. Under some conditions it is better, 

 before pouring in the solutions, to let the flasks stand for two 

 hours in the drying chamber at 150 0. The fluid is sterilised by 

 exposing the flasks containing it for at least two hours to a stream 

 of steam in the Koch's steam sterilising cylinder * (to be obtained 

 from H. Rohrbeck, Berlin). 



In especially accurate work the sterilisation in the cylinder is 

 repeated several days in succession. If in the first case spores 

 should remain alive, they may germinate in the period preceding 



* It is often sufficient merely to boil the fluid in the flasks for some time after 

 plugging with cotton wool. 



P.P. H 



