134 MINNESOTA BOTANICAL STUDIES. 



mention them. In his Anatomy of Plants (1687) he describes 

 them as galls, without stating definitely what caused their 

 development. De Candolle (1825) looked upon them as patho- 

 logical outgrowths, likewise refraining from expressing a defi- 

 nite opinion as regards their origin. Treviranus expressed the 

 opinion that they were undeveloped buds. 



Second Period : Collateral Investigations which led 

 to the Discovery of the True Nature of Root 

 Tubercles — From Lawes and Gilbert (i860) to Frank 



(1879). 



Perhaps criticism may be made for giving this as a period in 

 the history of the subject under discussion since the investiga- 

 tions referred to originally had absolutely no relationship to the 

 study of leguminous root nodules, nor did the investigators 

 about to be mentioned have any conception of the significance 

 of these root structures when they planned and began their re- 

 searches. 



The chief investigators of this period were Lawes and Gil- 

 bert of England and Hellriegel and Willfarth of Germany. 

 Their investigations pertained to the differences in the nitrogen 

 supply and nitrogen assimilation of certain plants, as grasses, 

 sugar beets, and leguminous plants. The final conclusion 

 reached by Hellriegel was that there was some definite signifi- 

 cant relationship between the root nodules and nitrogen assimi- 

 lation of leguminous plants. Immediately these root tubercles 

 were given marked attention which led to the discover)'' of their 

 characteristic contents, namely the bacteria, now more generally 

 known by the generic name rhizobia. 



Third Period : The Scientific Investigation of Legu- 

 minous Root Tubercles and Rhizobia — From Frank 

 (1879) to Schneider (1893). 



The first work of this period was really not done by Frank 

 (see Woronin and Hellriegel) but this investigator certainly 

 took the lead in doing active painstaking work in the study of 

 the leguminous root tubercles as well as the contained rhizobia. 

 Numerous other investigators of Germany, France, England 

 and America also did excellent work. Disputes and changes 

 of opinion were frequent. Not until the close of the period 

 were satisfactory conclusions reached regarding the true nature 

 of the root tubercles and the biological identity of the rhizobia. 



