NITROGEN ASSIMILATION 267 



He discusses the fungus which produces it, and shows that the 

 tips of the hyphae secrete a cellulose-dissolving ferment which 

 enables them to pierce the cell-walls of the host. This ferment 

 has since been described as cytase. He shows that its produc- 

 tion would determine the passage from a merely saprophytic to 

 a parasitic habit, and makes the suggestion that an organism 

 might be educated to pass from one to the other. 



An admirable research (1887) was on the formation of the 

 yellow dye obtained from "Persian berries" (Rhamnus infectorius}. 

 A dyer had found that uninjured berries afforded a poorer 

 colouring liquor than crushed. Gellatly had found, in 1851, 

 that they contained a glucoside, xanthorhamnin, which sulphuric 

 acid broke up into rhamnetin and grape-sugar. The problem 

 was to localise the ferment which did the work. Ward obtained 

 the unexpected result that it was confined to the raphe of the 

 seed. 



As early as 1883 Ward had attacked a problem which he 

 pursued at intervals for some years, and which was fraught with 

 consequences wholly unforeseen at the time. It had long been 

 known that leguminous plants almost invariably carried tuber- 

 cular swellings on their roots. The opinion had gradually gained 

 ground that they were due to the action of a parasite. Bacteria- 

 like corpuscles had been found in the cells of the tubercle, and 

 it was assumed that they had played some part in exciting the 

 growth of the latter. " No one had as yet succeeded in infecting 

 the roots and in producing the tubercles artificially." Ward 

 described, in a paper in the Phil. Trans, in 1887, how he had 

 accomplished this. He showed, in fact, that a definite organism 

 invades the roots from the soil, and finds its access by the 

 root-hairs. 



Lawes and Gilbert had long ago proved that the higher 

 plants are incapable of assimilating free nitrogen. Hellriegel 

 and Wilfarth had, however, shown in 1886 that leguminous 

 plants carry away more nitrogen from the soil than could be 

 accounted for. This Ward confirmed by his own pot-experi- 

 ments, and satisfied himself that the excess could only be derived 

 from the free nitrogen of the air. Hellriegel further concluded 

 that the tubercles played an essential part in the process. Ward 



