54 Parasitism. 



The specialisation of parasitism resulting in the evolution of biologic 

 forms is not confined to the Uredineae, but probably extends to parasitic 

 fungi generally, which frequent more than one host, and this has been 

 experimentally proved, particularly in the mildews or Erysiphaceae. Seve- 

 ral biologic forms may occur within a morphological species, so that it 

 will be necessary in the future, for the proper understanding of any such 

 species, not only to determine its limits by means of structural characters, 

 but also the special forms included in it with restricted powers of infec- 

 tion. Hitherto it has been generally assumed that the same parasitic fungus 

 occurring on two closely related host-plants would be mutually infective, 

 but this does not necessarily follow, even with different species of the 

 same genus. 



Neger 1 , in 1902, proved by numerous experiments that there were 

 biologic forms of Oidium for several species of Erysiphe, and Marchal 1 

 in the same year divided the one species of E. graminis into seven distinct 

 forms, using only the conidia for purposes of infection. He showed that 

 the biological form on barley was unable to infect wheat, oats, and rye, 

 and Salmon 2 carried the experiments a stage further by using the asco- 

 spores, which had the same restricted powers of infection. 



This difference in infective power is not due to any apparent structural 

 change in the fungus, for the form of E. graminis on the wheat is indis- 

 tinguishable, even under the microscope, from that on the barley, and yet 

 the form on barley cannot infect the wheat, nor can that on wheat infect 

 the barley. 



The difference, therefore, lies in the physiological peculiarities of the 

 host-plant, and it has been suggested that the cells of the leaf contain 

 an enzyme which is fatal to the growth of the haustorium of any other 

 form. But probably the action is reciprocal, and the germ-tube of the 

 fungus finds something in the particular host-plant which attracts it, and 

 is conducive to its growth. 





