Chapter VI — 97 — Parallelisms in Distribution 



is based on the fact that the spring generation of rust fungi form on 

 their plant hosts so-called aecia. These aecia develop only under 

 specific conditions existing on their usual host but not on any other 

 plants. Moreover, this specificity of rust fungi is so great that it some- 

 times enables us to distinguish between forms or even species of plants 

 in which with the naked eye we are unable to detect any difference in 

 morphological structure. The summer and fall generations of these 

 fungi are, however, much less specialized, and, consequently, the 

 uredospores and teliospores formed by them may develop on various 

 other plants. There thus may come about a facultative heteroecism of 

 the rust fungi that may later become permanent. If the new hosts 

 prove to be a more favorable substratum, the summer generations usu- 

 ally develop on them, and thus an originally autoecious parasite be- 

 comes heteroecious, although often, besides heteroecious forms, the 

 initial autoecious forms are preserved. Hence, in cases where there are 

 only summer and fall generations of a rust fungus on secondary hosts, 

 we may assume that they arose from autoecious forms in places where 

 the initial host grows, on which passed the entire cycle of development 

 of the fungus, including the spring aeciostage. 



According to Mordvilko, the cycles of rust fungi evolved in this 

 way: At first there were " lepto-forms " with several generations of 

 teliospores, among which might be distinguished the summer forms 

 and the forms that wintered over. Then came "brachy-forms", 

 when the summer teliospores were replaced by uredospores; and, 

 lastly, there arose the "eu-forms", when the first generation of uredo- 

 spores is replaced by aecia. Only "eu-forms" could develop heteroe- 

 cism. This occurred in the following manner: When in a given region 

 there arose new plants, or plants from other regions migrated to this 

 region, the rust fungi passed from their former hosts to new ones, pro- 

 vided the latter offered them favorable conditions for development, and 

 in time they became modified into new species and then genera. But 

 such a passing over to new hosts could be achieved only in the case of 

 "lepto-forms" and "brachy-forms". As regards "eu-forms", it would 

 be more complicated, since only the generations of uredospores and 

 teliospores could adapt themselves to the new hosts, while the aecia 

 could develop only on the primary hosts. At first, consequently, there 

 arose non-obligatory or facultative heteroecism and only later regular, 

 obligatory heteroecism. Hence, it would follow that the primary hosts 

 are more ancient than the secondary, and in most cases this has proved 

 to be so. 



If the foregoing is kept in mind, then, by the aid of the history of 

 rust fungi, it becomes possible also to elucidate certain problems in the 

 history of their hosts — to decide which are of a more ancient and which 

 of a more recent origin, which are indigenous to a given region and 

 which adventive. Cycles could evolve only in a temperate climate with 

 its seasons, while for the tropics with their even cUmate only "lepto- 

 forms" with a single type of spore are characteristic, all other forms of 

 rust fungi (with uredospores or aeciospores) undoubtedly being immi- 

 grants from regions with a temperate climate. A study of tropical rust 

 fungi from this point of view might also give much of value for an 

 elucidation of the history of their hosts. 



