Nc. 6. DEPARTMENT OF AGRICULTURE. 367 



straight rodlets lying parallel to the longer axis of the filaments 

 and embedded in a colorless matrix. This matrix did not consist of 

 cellulose, chitin, or any form of slime. The swellings, or vessicles 

 of Prazmowski, occurred at places where the rodlets had become 

 heaped up and eventually had burst liberating the rodlets. After 

 liberation from the filaments they became tranisformed into X, V 

 and Y shaped bacteroids. 



The exogenous theory is best represented by the researches of 

 Laurent in 1891,^^ and later by those of Atkinson in 1893.^^ 



Both hold that the filaments are not pouches containing bacteria, 

 but true homogenous filaments, which, as they enter the cells of the 

 central portion of the tubercle, show enlargements which afterwards, 

 by a process of budding, give rise to the branched bacteroid bodies. 



This view was also held by Ward^^ somewhat earlier, and all agree 

 in regarding the organism as a lowly organized fungus instead of a 

 bacterium whose filamentous portion corresponds to the mycelium, 

 and whose bacteroids are buds or gonidia thrown off from the latter. 

 For this fungus, Laurent adopts the name Rhisohium leguminosarum 

 originally proposed by Frank. 



Kegarding these two views the bulk of evidence inclines to the 

 exogenous theory, although there is opportunity for further study 

 before the matter is finally settled. 



In what has been already said it was assumed that the root 

 tubercles of leguminous plants are caused by an organism, com- 

 monly present in the soil, which infects the younger roofS and re- 

 sults in the formation of excrescenses or tubercles. 



In the earlier researches on the structure of the tubercle and their 

 contained organism it was assumed that the associated fungus was 

 the cause of the tubercle. But the first demonstrable proof of the 

 relation of soil organisms to the development of tubercles w'as due 

 almost simultaneously to HellriegeP* and to Ward,^^ in 1886. 



Hellriegel grew peas in sterilized sand free from nitrogen, but 

 to which the other mineral fertilizers had been added. The plants 

 grew until the stock of nitrogen in the seed was exhausted, when 

 they showed signs of starvation. To these starving plants were 

 added infusions of soil in which peas had previously been grow^n; in 

 one case this infusion was sterilized, and in the other not. When un- 

 sterilized soil infusion was used, the plants began to revive their 

 vigor, and continued to grow to maturity. On their roots numer- 

 ous tubercles developed. 



In the case of the plants watered with sterilized soil infusion, the 

 plants did not revive, and no tubercles formed. 



From this experiment and others of like character it was shown 

 that something, probably a living organism, existed in the soil in- 



