344 The Physiology of Plants BOOK in 



filled with a slimy colourless matrix in which bacteria-like 

 granules were embedded. In 1874 Eriksson found these 

 tubercles to be present almost always in all the genera of 

 the Papilionaceae, and regarded them as metamorphosed 

 lateral roots. He found fungal elements in them, existing 

 as hyphal filaments and as forms of Vibrio, but he was not 

 sure whether the two kinds were connected. 



In 1878 Kny ascertained that these nodules are not 

 developed upon plants cultivated in sterile nutrient fluids, 

 and the next year Frank showed that they do not occur 

 on the roots growing in sterilized soil. We are able, therefore, 

 to connect the presence of the fungus with the occurrence 

 of the tubercle. 



In his paper of 1879 Frank confirmed Eriksson as to the 

 presence of the hyphae and the bacteria-like bodies, and 

 was able to connect the two and to show that the latter 

 are budded off from the hyphae. 



We owe to Marshall Ward the most complete investiga- 

 tion of the structure of the tubercles, the mode of infection 

 of the plant, and the course of its development. His 

 researches were made in the years 1884-6, and appear to 

 have been suggested by the remarkable results that were 

 being obtained by Hellriegel and Wilfarth. Ward showed 

 that the nodules are absent from cultures of Vicia faba 

 in sterilized solutions, but can be made to appear with 

 great facility if pieces of chopped nodules are put in 

 contact with their root hairs. The growth is clearly 

 the result of infection by the fungus. The infecting agent 

 is normally in the soil and is taken up from it or from the 

 tubercles with the greatest readiness by water. Infection 

 takes place through the root hair, down which a filamentous 

 growth makes its way as soon as one of the bacterium-like 

 bodies comes into contact with it. After passing through 

 the root hair it traverses the cortex and enters the tissue, 

 which by hypertrophied growth becomes the young tubercle. 



