30 TRANSACTIONS AND PROCEEDINGS OF THE [Sess. LXI. 



THE BACTERIA OF LEGUMINOS^. 



Of the substances which are taken in by the roots of 

 plants, the most important in plant metabolism are the 

 nitrogenous compounds. Generally it may be stated that 

 while plants can and do take in oxygen as a free gas for 

 their respiration, they are quite unable to make use of free 

 nitrogen. The nitrogen so necessary in the plant economy 

 can only be absorbed in the combined state, say, as 

 nitrates. 



For long this was held to be true of all orders of plants, 

 until it was noticed that while in soils deficient in nitrogen 

 most plants came to nought, yet members of the order 

 Leguminosaj (peas, and clovers, and vetches) grew quite 

 well, and left the land after their growth richer in 

 nitrogen than before. When it came to be admitted that 

 leguminous plants must and did make use of the free 

 uncombined nitrogen, men cast about for an explanation of 

 their advantage as regards nitrogen, and several theories 

 were advanced before it came to be admitted that in the 

 nodule formation was to be found the real solution of the 

 problem. 



These warts, or swellings, or tubercles — as they are 

 variously called — were shown to bo little houses inside 

 whicli lowly plant forms lived in partnership with the pea, 

 or bean, or clover, deriving support from the plant, and 

 repaying this support by fixing the free nitrogen that 

 surrounded the roots, and forming it into compounds that 

 were absorbed by the leguminous [)lant. 



These swellings had been written of long before tlieir 

 real origin was discovered. As far back as 1687 

 Malpighi wrote of these as galls, and, coming nearer our 

 own time, they were looked upon by some as a sign of 

 a diseased condition, and by others as resulting from 

 nematode attack. Now we know that these nodules only 

 appear on the roots in case of infection with a bacterium, 

 the liacillus radicicola of Beyerinck {Ehizohium lc(juriii7ios- 

 aram, Frank). This bacillus is widely spread, and can be 

 obtained in quantity, for examination, if ])erfectly fresh 

 leguminous roots are laid in water and left to steep for a 

 few hours in a warm room. Amidst the bacteria, which 



