No. 6. DEPARTMENT OF AGRICULTURE. 369 



which later on produce buds which become bacterial and bacillary 

 bodies, and which eventually fill the cells of the central portion of 

 the tubercle. 



Such in brief is an outline of the biology of the root tubercle 

 organism. For a further exposition of the subject the reader should 

 consult the excellent paper of Atkinson. 



2. The Relation ofi Root Tubercles to INitrogen Assimilation. 



It has long been known that clovers enrich land, but the full 

 philosophy of the matter has not been fully understood until com- 

 paratively recent date. 



Lachmann,^® in 1S58, was probably the first to suspect that certain 

 nodular bodies seen upon the roots of legumes were in some way con- 

 nected with the absorption of nitrogen by the plant, but no experi- 

 mental proof of this assumption was given. 



In 1804, Kautenberg and Kuehn" gave the first inkling of this in 

 their work on water cultures. They noticed that with certain legu- 

 u:ens grown in water, nodules formed on the roots of only those 

 plants which were growing in a solution free from some compound of 

 nitrogen. The preseuce of nodule^/, therefore, seemed to be asso- 

 ciated W'ith an eii'ort on the part of the plant to obtain nitrogenous 

 food. 



DeVries,^^ a little earlier, observed that comparatively few nodules 

 developed on plants to which an abundance of nitric nitrogen was 

 supplied. Root tubercle development, therefore, seemed to be asso- 

 ciated with nitrogen hunger. But an exact demonstration of the 

 relation of root tubercles to nitrogen assimilation by the plant was 

 not made until 1886, when Hellriegel announced his important series 

 of researches. 



Hellriegel^- was able to grow peas and other legumens in pure sand 

 ab{<olutely devoid of nitrogen, but supplied wath other plant food. 

 The sand was sterilized so as to destroy all germ life therein. Peas 

 etc., grew in such a soil only under certain conditions, i. e., when 

 the soil was infected with an infusion containing the living germs of 

 root tubercles. Soils inoculated with sterilized infusions, or soils 

 not inoculated and previously sterile, failed to grow peas. 



In the infected soils the peas developed tubercles, and with their 

 development the plants thrived and attained their full maturity. 

 Since the soil was devoid of nitrogen, the plants could have gained 

 the nitrogen for their fruition from but one source, and that was the 

 atmosphere. 



Hellriegel thus, as a result of an elaborate series of experiments, 

 was able to announce positively that leguminous plants were able to 

 utilize the free nitrogen of the air. 

 24— G~1902 



