158 BACTERIA AND SOIL FERTILITY 



Wigand (1887) found that the tubercles contained true bac- 

 teria, and the following year these were obtained in pure cultures 

 by Beijernick. He found further that there were bacteria asso- 

 ciated with all tubercles, and altho the bacteria differed some- 

 what in the tubercles of different species of plants, still there were 

 certain constant characteristics to be seen in them all. He, there- 

 fore, regarded the tubercles as the result of the action of bac- 

 teria and gave to the organism producing the tubercles the name 

 of Bacillus raiicicola. He regarded the so-called bacteroids of 

 Woronin as degenerate involution forms of the bacteria, which 

 appeared only after the bacteria have lost their vigor. In a later 

 investigation, after isolating the bacteria and keeping them in 

 pure cultures for many months, he was able to produce the 

 tubercles at will by inoculating soils in which his plants were 

 growing with the pure cultures of the organisms. 



Later researches have confirmed all of HellriegeFs results and 

 show conclusively that if sufficient precautions are taken to steril- 

 ize the soil in which leguminous plants are grown no tubercles 

 result. It has been further shown that the tubercles grow on 

 plants developing both in the light and in the dark but are larger 

 on plants growing in the light; that they only appear on healthy 

 plants; that they are very few on plants growing in well-washed 

 sand; that if plants growing in sterilized soil be watered with 

 brook or river water, tubercles occasionally develop but never in 

 abundance; and that the infection of the roots occurs early in 

 the growth of the plant and cannot take place in the older 

 roots. 



The plant gets its nitrogen from the bacteria growing in the 

 tubercles and the bacteria get their carbohydrates from the plant. 

 What a beautiful example of the division of labor! Here we 

 have two friends living together, each performing the task which 

 it has become best suited to do through ages of specialization. The 

 legume with its broad, specially constructed leaves drinks in car- 

 bon dioxid and mysteriously gathers up the heat and light waves. 

 These enter the cell, the laboratory of that master chemist, 

 chlorophyll, where they are transformed into carbohydrates. 



