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 were most efficient. While these tests were useful, they 
were seldom entirely satisfactory, due to the frequency of chance 
inoculation of plants in either plats or pots by worms, insects, dust, 
and unsterilized water. 
It is obvious that tests of cultures should be carried on under 
sterile conditions, which should at the same time resemble as nearly 
as possible the soil and climatic conditions of nature. Although nod- 
ules have been produced frequently by growing sterilized inoculated 
seedlings upon sterilized agar media in flasks,! this method of testing 
cultures is undesirable on account of abnormal conditions. The use 
ef jars containing small quantities of sterilized soil has also been 
attempted,? but without success. During the last year numerous ex- 
periments have been conducted with a special type of jar supplied 
with a tubular neck near the bottom, and it is believed that with this 
simple apparatus accurate tests of the inoculating power of the bac- 
teria may be made at frequent intervals. 
TESTING THE INOCULATING POWER. 
A jar of the type shown in figure 1 is charged first with a layer of 
cinders or broken marble and then with coarse sand moistened 
with Sachs’s solution lacking in nitrogen compounds; the side 
tubulature is plugged with cotton wool; a narrow layer of cot- 
ton wool is drawn 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 1s 
ready for use. To avoid cracking the jar the autoclave must be 
raised to the sterilizing temperature very gradually, and at the end 
of the sterilizing period the temperature must be allowed to fall very 
gradually. At 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, with practically no danger of contamination. Black paper is 
then wrapped around the lower 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 within the jar, since both top 
and bottom offer opportunity for the passage of air through the cot- 
1 Harrison, F. C., and Barlow, B. The nodule organism of the Leguminose—its isola- 
tion, cultivation, identification, and commercial application. Centralblatt fiir Bakteriolo- 
gie [etc.], Abt. 2; Bd. 19, No. 7/9, p. 264—272, 1907. 
Kellerman, Karl F. The present status of soil inoculation. Centralblatt fiir Bakteri- 
ologie [etc.], Abt. 2, Bd. 34, No. 1/3, p. 42-50, 1912. 
2 Wilson, J. K., and Harding, H. A. Method of keeping bacteria from growing plants. 
Science, n. s., v. 33, no. 849, p. 545, 1911. (Abstract.) 
[Cir. 120] 
