326 Professor Marshall Ward, On the Question of 
it meet to cope with those offered by 0, and we term B sus- 
ceptible and 0 immune. 
Or, finally, it is conceivable that the conditions— substantial 
or otherwise — of its original substratum, a living host x , have so 
affected the fungus that it is unable to avail itself of the favour- 
able conditions offered by a species or variety y, and so does not 
successfully infect it, but is quite capable of infecting z, and 
benefiting by the substances, or conditions, offered by the cells 
of the latter. 
In discussing these questions, it seems to me that plant- 
pathologists have not sufficiently clearly borne in mind the 
probability that two distinct classes of phenomena are here con- 
cerned — (1) the activities of the host, on the one hand, towards 
the fungus may be attractive or repulsive : in health they should 
be the latter. (2) The relations of the fungus towards the host 
may be aggressive or passive : they are usually aggressive. 
Theoretically any or all of these possibilities may exist, and 
the resultant may be the outcome of a very complex condition 
of affairs, viz. attractions and repulsions of the fungus towards 
and from the host, and attractions and repulsions of the host 
towards and from the fungus. 
Moreover, as we know from the recent advances in the study 
of enzymes and of chemotactic substances, of toxins and anti- 
toxins, such substances and conditions as I have postulated above 
do actually occur in the living cells, and there is not only no 
absurdity but, on the contrary, every show of probability that — 
since the structural features elucidated by the microscope are not 
responsible for the phenomena of immunity and susceptibility on 
the part of the host, or of capacity or incapacity to infect on the 
' ; part of the fungus — it is in the domain of the invisible biological 
properties of the living cell that we must expect the phenomena 
to reside. 
This brings me to the consideration of some remarkable re- 
semblances, or coincidences, between the behaviour of these 
fungus-spores towards their host, and, reciprocally of the host to 
the parasite, and that of the pollen and stigma the one towards 
the other. 
It is obviously not straining the facts to compare the physio- 
logical behaviour of a uredospore and its germ-tube towards the 
tissues of a leaf, with that of a pollen-grain (which is also a spore) 
and pollen-tube towards the tissues of the stigma and style on 
which it germinates and into which it penetrates. 
Just as the hyphse of parasitic fungi are attracted by chemo- 
tactic substances 1 so are the pollen-tubes 2 , and even if we did not 
1 Miyoshi, Jahrb. f. wiss. Bot. 1895, B. xxvm. p. 269. 
2 Miyoshi, Bot, Zeitg. 1894, p. 23. 
