I] SPECIALIZATION 2i 



ear, where they produce the condition known as otomycosis aspergillana, but 

 they develop on the secretions and not in the hving cells, so that, although 

 they give rise to a disease, they are not ti'ue parasites. 



Similarly Synchytrimn diirewn infects all sorts of dicotyledons and Phyl- 

 lactiiiia Corylea occurs on the leaves of many trees. 



In other cases the range is somewhat narrowed ; many species of Hydnum 

 are found only in fir woods, Pyronema confluens and a number of other fungi 

 occur in nature only on burnt ground, Onygena equina is restricted to the 

 hoofs and horns of various animals, and several species {Sordaria macrospora, 

 Podospora copropliila, Ascobolus inimersus, Ascophanus equinus, etc.) develop 

 upon different kinds of dung but on no other substratum. Some of the 

 coprophilous fungi on the othenhand may appear in addition on other sub- 

 stances, especially on those rich in cellulose — Ascophanus carnens has been 

 recorded upon paper or rope, Gymnoascus Reesii on wasps' nests, Xylaria 

 Tulasnei on soil. 



Parasites again may be limited to hosts of a particular family; Cystopus 

 candidiis to the Cruciferae, Claviceps purpurea to grasses, Ustilagd violacea 

 to the Caryophyllaceae. 



Still greater restriction is observed in the case of such saprophytes as 

 Erinella apala on the dead stems of species of Juncus and Pilacre fagi'nea 

 on rotten beech wood, and in the case of the parasites which attack the 

 species of a single genus, such a.s Rhytisma Acerinum on Acer, Poly stigma 

 rubrum on Prunus, Peronospora Euphorbiae on Euphorbia. 



Even more definite is the specialization of fungi which are capable of 

 obtaining nutriment only from a single species ; this occasionally happens 

 among saprophytes, such as Dasyscypha clandestina on dry stems of Rubus 

 idaeus; it may be surmised that in many such cases new habitats will ulti- 

 mately be brought to light. 



Among parasites restriction to a single species is very common ; Sclerotinia 

 tuberosa forms its sclerotium attached to the rhizome oi Anemone nemorosa, 

 Exoascus Betulae develops on Betula alba, and Ustilago Maydis on Zea Mays. 



De Bary gave to this state the name monoxeny, in contradistinction to 

 the term polyxeny, which he applied to cases where hosts of several different 

 species may be attacked. Every sort of intermediate grade may exist between 

 those outlined above ; parasites may attack only two or three closely related 

 members of a genus {Exobasidium Rhododendri on Rhododendron hirsutum 

 and R. ferrugineum), they may attack a genus and one or two species 

 belonging to related genera, they may be capable of development on certain 

 genera of a family and not on the rest, or again, while commonly restricted 

 to a particular family, they may occur also on members of neighbouring 

 croups ; thus Phytophthora infestans is found usually on the Solanaceae and 

 exceptionally on the scrophulariaceous species Anthocercis viscosa and 

 Schizanthus Grahami. 



