326 Professor Marshall Ward, On the Question of 



it meet to cope with those offered by C, and we term B sus- 

 ceptible and C 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 hyphas 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. 



