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on their roots, and experiments of Hellriegel and others showed 
a parallel between the development of these root tubercles and the 
power of obtaining nitrogen in large quantities. It was Hellriegel, 
also, who first demonstrated that these root tubercles were abnor- 
mal products on the roots of the pea, produced there by certain 
organisms in the soil. Hellriegel found, for instance, that peas 
growing in sterilized soil produced no tubercles and fixed no ni- 
trogen, while peas growing in a similar sterilized soil but watered 
with water in which ordinary soil had been standing for awhile 
(soil infusion), did develop tubercles and fixed nitrogen. The 
same soil infusion when sterilized was not able to cause the pro- 
duction of tubercles. From all of this it was evident that the 
tubercles were produced upon the roots of the peas through the 
agency of some of the organisms in the soil. 
A microscopic study of these root tubercles soon attested the 
same conclusion. It was found that the tuberacles were filled with 
small organisms related to bacteria, and that the development of 
the tubercles was parallel to the developement of these organisms. 
The organisms are somewhat different from normal bacteria and 
have been called bacteroids. 
According to the investigations of Prazmowski in 1890, the 
development and growth of the tubercles are as follows: Bacter- 
zum radiciola lives normally in the earth and collects in numbers 
on the outside of the roots of various legumes. Some of the or- 
ganisms succeed in forcing their way into the tissues of the young 
roots, though they are not able to pierce the older roots. Fora 
while they may remain in the root as free bacteria, but the plant 
plasma seems to exert an injurious influence upon them, for very 
soon a thin membrane is formed around the bacteria masses, in- 
closing them like a pouch, Prazmowski thinks that this mem- 
brane is a product of the bacteria themselves, formed for the pur- 
pose of protecting them from the injurious action of the plant tis- 
sue. The bacteria, which do not succeed in getting into one of 
these pouches, soon cease to grow and degenerate into irregular 
forms called bacteroids. The bulk of the bacteria, however, be- 
come inclosed in the membrane, after which they continue their 
growth with much vigor. The pouches begin to grow in thread- 
like masses, and these make their way among the cells of the root. 
