L6 HETEECECISM 



adapl itself to new conditions and thus tends to perpetuate the 

 race, while the change of host, which is equivalent to a change 

 nt' diet, may very possibly tend to an increase of vigour in the 

 hhIi\ [dual. 



The former statement, however, must not be taken to refer 

 to all cases of hetercecism, since there are instances, e.g. in 

 species of Goleosporium, which cannol be explained on this 

 ground. 



The reason why P. Caricis has been taken as the typical 

 rivdine, instead of the usual P. gra minis, is that the aecidium 

 of the latter is now very rarely found in this country and is 

 t herefore not available for demonstration, while that of P. Car ins 

 is common in all suitable localities. Even if not existing in 

 any place, it can be readily introduced if the three prerequisites 

 are at hand: — a pond bordered by Garex paludosa and by 

 Urtica dioica, and also a quantity of the leaves of Garex 

 infested by the parasite. The Nettle and the Sedge are nol 

 injured appreciably by the disease, nor would it be of much 

 consequence if they were. To introduce the fungus into the 

 new locality, it is only necessary to obtain a bundle of the 

 required leaves (say about 500) from some place where the 

 Pucdnia exists, in January or February, and lay them on 

 the ground where a patch of nettles is known to occur. The 

 latter will be seen in spring to be beautified by the aecidium, 

 and in the summer the disease will spread to the surrounding 

 sedges (see Grove, Journ. But. L913, p. 42). 



