62 BRITISH FUNGI 



by the numerous kinds of funj^i known as wound-parasites, that is, 

 fungi that are incapable of directly infecting a living host-plant 

 througli an unbroken surface, but wiiich, when placed on a wounded 

 portion of the host,, first live on the injured tissue, and gradually 

 pass on to the li\'ing tissues, and act in every way as true parasites. 

 To this class belong most of the large, woody bracket-fungi {Poly- 

 poriis and Fonies), commonly met with on the trunks of living trees, 

 also the beefsteak fungus {Fistiilina), and many agarics. The 

 wound-parasites gradual!}' pass into another group of fungi that 

 may be termed occasional parasites, that is, fungi whicli, as o])por- 

 tunity offers, can live as either sapropltytes or parasites. These 

 fungi are numerous and but little understood in their relation to 

 sapropliytism or parasitism, and every now and again for some 

 unexplained reason become rampant parasites, and occasion 

 serious loss if the hosts happen to be cultivated plants. 



A third proof tliat fungi have become parasites by steps as it were, 

 is the fact that fungi known only as saprophytes can be educated 

 to become rampant parasites, by sowing the spores on a living host- 

 plant, to which is added some of the sap of tlie saprophytic material 

 on which the fungus normally grows. When this method is repeated 

 se\-eral times, always using the spores of the preceding generation, 

 the fungus gradually acquires a new taste, and in course of time 

 the spores will germinate and attack the new host-plant without 

 the assistance of any of the food-material that it required as a 

 saproph^'te. Hence it would appear that in many instances the 

 habit of parasitism is the outcome of circumstances. T!ie spores of 

 a saprophytic fungus happen to be blown on the \\'ounded portion 

 of a living plant. The spores germinate on the dead tissue of the 

 wounded portion, and gradually pass on into the hving portion. 

 By such means we get strains of fungi ; one specimen happens, 

 through a mere accident, to gain access through a wound to living 

 tissue, and acquires tlie habit of parasitism, which is inherited by its 

 offspring. Another individual of the same species, lacking oppor- 

 tunity, remains a pure saprophyte. 



Parasitism has been reduced to a fine art in many of tlie most 

 advanced groups of parasitic fungi, as the rusts of cereals. Here 

 we have what liave been termed " biologic species," that is, species 

 which possess no structural or morphological differences, one from 

 another, but wliich can be readily recognized by biological pro- 

 perties. Such biologic species originated from a common species 

 or ancestor, whose structural- peculiarities they retain, but certain 

 members have become so specialized that, as parasites, they can 

 only infect and live upon a single kind of host-plant. Another lot 

 of the same species in like manner has become so highl}' specialized 

 that it can also only infect one special species of host-plant, and so 

 on. Thus biologic species simj^ily signify so many forms belonging 

 to a common species that ha\e struck out a line for themselves, 



