430 Transactions. — Botany. 



was thrown on the question. Some two hundred years ago 

 Malpighi, an Itahan savant, drew attention to certain nodules 

 or tubercles that grew on the roots of leguminous plants ; and 

 at a later time Linnaeus described these nodules on the roots 

 of LatJiyrus (the sweet-pea). From this time the nodules con- 

 tinued to attract the occasional notice of botanists, but nothing 

 of importance was learned about them for many years. In 

 1856 Lachmann investigated their minute structure, and 

 much discussion ensued, but with no real understanding of 

 the facts. In 1866 Woronin made the important discovery 

 that the root - tubercles contained great numbers of minute 

 organisms, since named "bacteroids" — i.e., bacterium - like 

 bodies. Things were thus getting into train for a true com- 

 prehension of the role played by the root-tubercles in the 

 nutrition of leguminous plants ; but ten more years had to 

 ■elapse before the connection between the activity of the root- 

 tubercles with the associated bacteroids on the one hand, and 

 the abnormal fixation of nitrogen by leguminous plants on the 

 other, was conceived and investigated. 



In November, 1886, two German investigators, Professor 

 Hellriegel and Dr. Wilfarth, who had been engaged for a con- 

 siderable time on a wide and fruitful investigation of the 

 nitrogenous nutrition of gramineous, leguminous, and other 

 agricultural plants, published their first important results. To 

 these I shall refer at some length, not only on account of their 

 intrinsic importance, but also because they afford excellent 

 examples of scientific method, of skilfully-directed interroga- 

 tion of nature, and of the cautious and logical interpretation of 

 experimental I'esults. 



To ascertain the sources of the nitrogenous food of plants 

 these inquirers sowed and raised a variety of gramineous 

 plants (including cereals and grasses) in a soil deprived of 

 nitrogen and protected from rain, but to which all other 

 mineral substances necessary for the healthy nutrition of 

 plants had been added in proper quantities. The seedlings 

 developed normally until the third leaf appeared, when the 

 reserves of food contained in the seeds were exhausted. At 

 this point growth ceased suddenly. The plants did not die ; 

 they lived almost as long as normal plants, but their vegeta- 

 tion was dwarfed. The stunted plants developed stunted and 

 miserable organs, even barren ears, and struggled through the 

 season. Their total dry weight was found to have increased 

 very little beyond that of the seed, and the increase was of 

 non-nitrogenous substances only. When nitrates were added 

 to the soils as soon as the arrest of growth set in, normal 

 growth was soon resumed, and if sufficient nitrates were added 

 it continued without check to full maturity. If, on the other 

 hand, the nitrates were insufficient, a gradual passage to the 



