Parasitism. 53 



host, together with the results of infection experiments, have considerably 

 modified our views as to the limits of species in such fungi. 



Eriksson l , in dealing, with cereal rusts particularly, found that they 

 were not liable to infect indiscriminately the different cereals, but were 

 confined to one, or, at most, a few closely-allied host-plants, and to this 

 phenomenon, so widespread among parasitic fungi, he applied the appro- 

 priate name of specialisation. 



As examples of specialisation among heteroecious rust-fungi may be 

 given those of Puccima coronata, Corda, and P. graminis, Pers. Klebahn } 

 proved by infection experiments that the crown rust on Dactylis glomerata 

 and other grasses only produced its aecidium on Frangula alnus, while that 

 on Lolium perenne required for its aecidial host Rhamnus catliartica. Hence 

 the old species was split up into two, which can also be separated by mor- 

 phological characters. Eriksson 1 also proved in 1894 that the well- 

 known and much-investigated species of Puccima graminis could be split 

 up into a series of forms, all of which agreed in producing aecidia on the 

 barberry, but differed in the uredo and -teleutospore generations, only being 

 able to infect special host-plants. Puccinia dispersa, Eriks., was proved 

 to be an independent species, with uredo and teleutospores on rye, and its 

 aecidia on species of Anchusa. Included in this were a number of forms 

 which had no known aecidial stage, and they were afterwards separated 

 and raised to specific rank as P. triticina, P. bromina, P. agro-pyrina, &c. 



If a general view be taken of this phenomenon it is found that when 

 two closely-related species, say, A and B, are attacked by a rust-fungus, 

 the one on A will not infect B, and that on B will not infect A, even although 

 the two fungi are the same species, regarded from a morphological point of 

 view. There must, however, be some adaptation between the host and the 

 fungus, so that the latter is attracted towards the one host and repelled by 

 the other. But it has been shown by Ward l that occasionally a spore from 

 A may gain a footing on B, and once having done this it can continue to 

 infect B, since it has now become adapted to it. 



According to the same observer, parasites may be educated to attack 

 fresh plants by means of what he calls bridging-species. Thus, while the 

 parasite on A may be unable to infect B, it mav be able easily to infect 

 a related species C, and after establishing itself on C it may then have 

 the power to infect B so that C becomes the bridging species from A to B, 

 Massee 9 has also shown that a parasitic fungus can be led to attack a 

 new host-plant by injecting a substance positively chemotactic to the fungus 

 into the tissues of the living leaf. 



Parasitism is thus an acquired habit, and, generally speaking, it be- 

 comes specialised, because only in certain plants are the substances present 

 which attract the fungi, while in others there are also certain substances 

 which repel, and thus prevent their germ-tubes gaining a footing in the 

 tissues. 



But in contrast to this specialisation, there occurs in a few species what 

 may be called general parasitism, where the parasitic fungus is able to in- 

 fect host-plants widely separated in their affinities. 'Fischer 3 and 

 Klebahn 1 have shown that Cronartium asclepiadeum can attack plants 

 belonging to such distantly related families as Ranunculaceae and Scrophu- 

 lariaceae, as well as Asclepiadeae, so that it has become necessary to unite 

 under this name, species which were formerly separated on account of the 

 difference of host-plant. 



Chemotaxis can hardly be held accountable for such a widely-divergent 

 distribution of host-plants, and although it does not clear the matter^ up, 

 it may be referred to the " internal developmental tendencies 

 Klebahn l until a better explanation is forthcoming. 



