I] SPECIALIZATION 23 



many fruitful mycological discoveries this was foreshadowed by de Bary, 

 who in 1863, noticed that the structural differences between the aecidia of 

 Chrysoniyxa Rhododendri and C. Ledi were so slight that he regarded these 

 as " rather biological than morphological species." 



Thirty years later Eriksson recognized that the rust of wheat, Puccinia 

 Graminis, which infects v/heat, barley, rye, oats and various wild grasses, 

 is a collective species, consisting of a number of biological forms which, 

 though they differ in no recognizable structural character, yet differ in the 

 powers of infection of their spores, since uredospores grown on wheat are 

 incapable of directly infecting rye, barley or oats, those on oats cannot 

 directly infect wheat, rye or barley, and those on ,barley and rye, though 

 they can infect both rye and barley, will not develop if sown on oats or 

 wheat. Now Puccinia Graminis is a heteroecious species producing two 

 kinds of spores (uredospores and teleutospores) on grasses, and aecidiospores 

 in cluster-cups on the barberry. But, though all the different biological forms 

 alike develop their cluster-cups on the barberrry, Eriksson found that they 

 remained constitutionally distinct, for aecidiospores derived from the form 

 upon oats proved capable of infecting among cereals only oats, aecidiospores 

 from the form upon rye or barley, only rye or barley and so on. 



In other words each form of Puccinia Graminis is so closely adapted to 

 the particular cereal on which it occurs that its spores can only attack 

 successfully and directly that particular graminaceous host or a limited 

 number of its immediate allies. 



Marshall Ward, working with the uredospores of Puccinia dispersa, made 

 clear that the susceptibility or immunity of the host does not depend on 

 structural characters, and suggested rather the existence of enzymes or 

 toxins or both in the cells of the fungus, and of antitoxins or similar sub- 

 stances in the cells of the host. 



This hypothesis has been greatly strengthened by the work of Marchal 

 and of Salmon on the Erysiphaceae or mildews in which group both the 

 conidia and ascospores of biological species are similarly specialized in 

 their powers of infection. Not only are there no structural peculiarities in 

 the resistant hosts of these fungi, but Salmon was able, by suitable treatment, 

 to break down their resistance. This may be achieved in various ways: 

 (i) a minute piece of tissue, including the epidermis and the greater part of 

 the mesophyll, is cut with a razor from one side of the leaf and spores are 

 sown on the opposite side; (2) the leaf to be inoculated is touched for a few 

 seconds on the upper surface with a red-hot knife and the spores are sown 

 on the lower surface opposite the burnt spot. 



The result of such treatment is the ready infection of host species 



to cause infection except of a flower directly inoculated. Biological species have also been identified 

 among smuts, and by Diedicke in Pleospora. 



