22 INTRODUCTION [ch. 



In all the cases of parasitism considered above, the fungus, whether 

 monoxenous or polyxenous, is capable of passing through its whole existence 

 on a single host. A different type of specialization appears in those fungi 

 which require two host species for the completion of their life-history, and 

 produce characteristic spores on each. Such forms are said tobe heteroecious 

 in contradistinction to the autoecious species on a single host. 



Heteroecism. Heteroecious species may be either monoxenous or poly- 

 xenous, that is to say they may be limited in relation to either or both their 

 stages to a single species of host, or they may be capable of occurring on a 

 number of different forms. Heteroecism is known in Sclerotinia Ledi but 

 in no other fungi except the Uredinales or rusts, and its full consideration 

 must be postponed till that group has been described. It may be considered 

 here as one of the possible alternatives confronting a parasite on a host 

 which dies down early in the year. Under such circumstances the parasite 

 may continue its development on the dead tissue, it may await in the form 

 of resting spores the reappearance of its host, or its spores may prove capable 

 of germinating on a new host species, and it may carry on its development 

 as a parasite and reinfect its original host next spring, thus becoming 

 heteroecious. It must be noticed, however, that by no means all the 

 existing spring or summer hosts of heteroecious fungi die down early 

 in the year, so that other and possibly secondary factors will have to be 

 considered. 



Biological species. There is no doubt that within wide or narrow limits 

 related host plants are apt to show common susceptibility to infection ; this 

 is well exemplified by Puccinia Malvacearum which was first observed in this 

 country on cultivated hollyhocks in 1873 and has since established itself on 

 the indigenous species of Malva, Altliea and Lavatera as well as in green- 

 houses on Abut Hon. 



Common susceptibility may even be used as a criterion of relationship, 

 so that liability to the attacks of Piptoceplialis Freseniana, the obligate 

 parasite of the Mucorales, has afforded a means of distinguishing new 

 members of that group. 



These facts point to a definite adaptation on the part of the fungus to 

 its habitat; this adaptation may be very simple, a species, for example, would 

 not be likely to occur on dung unless its spores could pass uninjured through 

 the alimentary canal of an animal, or it may reach the complexity of a 

 delicately balanced reaction between host and fungus, as in some of the 

 mycorhiza described in the preceding pages. 



Specialization has been most fully investigated and has possibly reached 



its highest levels in the adaptation of various rusts and mildews to their 



. graminaceous hosts, but it is shown in varying degrees by other fungi 1 . Like 



1 Wormald has shown that there are two biological species of Monilia (Sclerotinia) cinerea, one 

 of which produces a Blossom Wilt and Canker Disease on the apple tree while the other is unable 



