NATL'KAL AND AK1IFH IAL l.\KK< TI< >N. 51 



The exact proof, however, is best obtained by meaii> of e\p,-ri- 

 mental infection. 



With many parasites the sporocarps are normally developed 

 saprophytically on a dead substratum, so that if parasitism 

 be suspected it can only be proved by infection. Thus the 

 perithecia of N-frui dnnabarina develop only after the death 

 of the plant-organ, which the funuus attacked when alive. The/ 

 more complex reproductive organs of many fungi are developed 

 onlv on dead remains of the ho>t, while on living or dviii" 



O i 



parts one finds various forms of eonidia of doubtful relation- 

 ~hip. In many cases it has been possible, by means of artificial 

 culture alone, or combined with artificial infection, to prove 

 various forms of reproductive organs to be stages in the life 

 of the same fungus. 



\Vhen a group of fungi contains both saprophytes and jiarasites, 

 it is often necessary to determine whether some species is para- 

 sitic or purely saprophytic. This is particularly the case with the 

 .groups of Pyrenomycetes, Discomycetes, Hymeiiomycetes, several 

 groups of the lower Funui, the Bacteria, and Myxomycetes. It 

 is unnecessary, however, with tin.- I'redineae, l">tila^ini-ae, I'.-i- 

 onosporeae, Exoasceae, and other groups known to contain 

 parasites exclusively. 



Hut even in the>e last-mentioned groups experimental in- 

 fection is neces-ary for obtaining information on other pohn<. 

 The reproductive organs of I'redineae cannot be reared in 

 artificial .-solutions, BO iliat their cultivation mu.M be carried nut 

 on the living host-plant. In this way alone can \\v ascertain 

 the relationship of uredospoivs, leleiito-^pures, and aecidial-forms, 

 where any doubt occurs as to their belon-iiii; to the .same 

 ~pecie<. Infection becdiues particularly valuable when one has 

 to investigate heteroecious rredineae, whose various lorms of 



reproductive Organs inhabit several host-plants. Thus ii was 



by ni'Miis of infeetiuii that I >c I'.ary discovered the cnniieetiun of 

 Aecidium berberidis n the liai-berry. and I'ni-i-inin </,-<i///i//is mi 

 Cereals; likewise Hartig, the relationship of .)/, /<i/itj>*,,,;i <;,/>- 

 pertiana on cowberry with Aecidium <,,!, nn nun on needli-s of 

 .-ilver fir. There Mill remain many aeeidia, beleiltospores, and 

 Uredospores, \\hosi- relateil forms have nt yet been found. 



Infections are al-o Qecessary i" determine tin- species of a 



Inn-jus. It has lieen found, for example, that Gymnas^ // m 



