4 CIRCULAR NO. 120, BUREAU OF PLANT INDUSTRY. 



of leguminous crops in the field has been relied on to show -which 

 strains of bacteria had but a moderate or low inoculating power 

 and which Avere most efficient. While these tests were useful, they 

 AA'ere seldom entirely satisfactory, due to the frequency of chance 

 inoculation of plants in either plats or pots by worms, insects, dust, 

 and unsterilized Avater. 



It is obvious that tests of cultures should be carried on under 

 sterile conditions, Avhich should at the same time resemble as nearly 

 as possible the soil and climatic conditions of nature. Although nod- 

 ules haA-e been produced frequently by growing sterilized inoculated 

 seedlings upon sterilized agar media in flasks,' this method of testing 

 cultures is undesirable on account of abnornuil conditions. The use 

 of jars containing snudl quantities of sterilized soil has also been 

 attempted,^ but Avithout success. During the last year numerous ex- 

 periments luiA^e been conducted with a special type of jar supplied 

 Avitli a tubular neck near the bottom, and it is belieA^ed that AA'ith this 

 simple apparatus accurate tests of the inoculating power of the bac- 

 teria may be made at frequent interA^als. 



TESTING THE INOCULATING POAVER. 



A jar of the type shoAvn in figure 1 is charged first with a layer of 

 cinders or broken marble and then with coarse sand moistened 

 AA'ith Sachs's solution lacking in nitrogen compounds; the side 

 tubulature is plugged Avith cotton aa'ooI ; a narrow layer of cot- 

 ton Avool is draAvn around the jar near the top; and over this a 

 snugly fitting beaker is carefully slipped. The jar is now steril- 

 ized in an autoclave at 20 pounds pressure for 30 minutes and is 

 ready for use. To avoid cracking the jar the autoclaA'e must be 

 raised to the sterilizing temperature A'ery gradually, and at the end 

 of the sterilizing period the temperature must be alloAved to fall A^ery 

 gradually. .Vt least three hours should be consumed in this process. 

 By using ordinary bacteriological precautions it is possible to intro- 

 duce sterilized seeds through the side tubulature and later inoculate 

 them. Avith practically no danger of contamination. Black paper is 

 then Avrapped around the loAver part of the jar to protect the growing 

 roots from light. A slight circulation of air results from the proc- 

 esses of respiration and photosynthesis Avithin the jar, since both top 

 and bottom ofi'er opportunity for the passage of air through the cot- 



1 Harrison, F. c"., and Karlow, B. The nodnlo organism of tlio I.c:ruminosa^ — its isola- 

 tion, cultivation, identification, and commercial application. Centralblatt fiir liakteriolo- 

 gie [etc.], Abt. 2, Bd. 10, No. 7/9, p. 264-272. 1907. 



Kellerman, Karl F. The present status of soil inoculalion. Centralblatt fiir Baktcri- 

 olofrie [etc. J, Abt. 2, Bd. :14, No. l/:{, p. 42-.50, ItlTJ. 



- Wilson, .1. K., and Ilardinj?, II. A. Method of keepiug bacteria from growing plants. 

 Science, n. s., v. :Ju, no. 849, p. 545, 1911. (Abstract.) 

 [Cir. 120] 



