v1 Obituary Notices of Fellows deceased. 
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-experiments 
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 had no doubt that the bacteroids 
were the channel of supply. But he failed to get any proof that they could 
assimilate free nitrogen outside the plant. He suggested that their symbiosis 
might be an essential condition, and was obliged finally to leave it an open 
question whether the cells of the tubercles or the bacteroids were the active 
agents in nitrogen assimilation. He had already stated in 1887 that it is 
very probable that the bacteroids “may be of extreme importance in agri- 
culture.” But he was never satisfied with anything short of the strictest 
proof. 
In 1890 Ward was invited to deliver the Croonian Lecture. He chose for 
his subject the relation between host and parasite in plant disease. He 
defined disease in its most generalised form as “the outcome of a want of 
balance in the struggle for existence.” But the particular problem to which 
he addressed himself was the way in which the balance is turned when one 
organism is invaded by another. This is the most common type of disease 
in plants and a not infrequent one in animals. The first result reached 
was identical with that of Pasteur for the latter; the normal organism is 
intrinsically resistant to disease. It is an immediate inference that natural 
selection would make it so. Ward then discusses very clearly the physio- 
logical conditions of susceptibility, which he shows to be a deviation from 
the normal. He had already indicated this in the case of Entyloma. The 
epidemic phase is reached when the environment is unfavourable to the host 
but not so or even favourable to the parasite. He then attacks the more 
obscure case where there is no obvious susceptibility. This, he finds, resolves 
itself into a mere case of the struggle for existence: “a struggle between the 
hypha of the fungus and the cells of the host.” It is more subtle in its 
operation but of the same order of ruthlessness as the ravages of a carnivore. 
Ward’s account of the struggle is almost dramatic. The cellulose “out- 
works ” are first broken down, as he had previously shown, by a secreted 
ferment. The “real tug of war” comes when the hypha is face to face with 
the ectoplasm. Its resistance is at once overcome by flooding it with a 
poison, probably oxalic acid. 
