E. B. FRED ■ 625 



connected to a water pump, preferably with a suction flask between 

 E and the pump. 



The seeds are placed in D and the suction pump is started. Clamp 

 1 is opened and the mercuric chloride allowed to run into D until 

 there is enough of the solution to cover the seeds, then Clamp 1 is 

 closed. The reduced pressure removes the air bubbles from the seed- 

 coats and thus permits the mercuric chloride solution to come in 

 closer contact with the surface of the seed. The time required to 

 destroy the bacteria, as well as spores, depends upon many factors: 

 nature of seed-coat, kind of bacteria, etc. Field peas sterilized for 

 3 minutes in a 0.25 per cent mercuric chloride solution were found 

 to be entirely free of bacteria. Smaller seeds, as clover and alfalfa, 

 require only from 1 to 2 minutes to be sterilized. 



To remove the mercuric chloride, the bottle containing the seeds is 

 inverted. When the liquid is removed, bottle D is returned to its 

 original position and water from C allowed to run in until the seeds 

 are covered. The seeds are then washed several times with the 

 sterilized water from C to remove the mercuric chloride. The amount 

 of water used should be regulated so that C will be empty after the 

 final washing. When the water is removed from D, clamp 3 is closed 

 and clamp 2 is opened slowly in order to restore atmospheric pres- 

 sure in D. Bottle D is removed and the seeds are transferred directly 

 to a sterile Petri dish. 



The importance of growing plants in the absence of bacteria is well 

 recognized and many different types of apparatus have been designed 

 for this purpose. 



In 1898 Lauck^ described a method for the growth of plants without 

 bacteria, but unfortunately the apparatus he used is very compli- 

 cated, expensive, and difficult to handle. It is recommended for the 

 growth of large sized plants. Schulze^ in 1901 devised a vessel for 

 growing large plants free of bacteria which has many advantages. 

 The methods described by Petri^ and by Schmidt*' were designed 

 especially for young seedlings and, therefore, are not included in this 



3 Lauck, H., Centr. BakterioL, 2te AM., 1898, iv, 706. 



* Schulze, C, Landw. Jahrb., 1901, xxx, 327. 



5 Petri, L., Centr. BakterioL, 2te Abt., 1909, xxiv, 146. 



« Schmidt, E. W., Centr. BakterioL, 2te AM., 1910, xxv, 426. 



