UREDINALES 583 



Furthermore, it is noteworthy that normally not an aecial but a 

 uredinial mycelium arises from aeciospores. Similarly from teliospores, 

 only basidia and later aecia may be formed. The gametophyte, therefore, 

 consists only of a short generation, that from the basidiospore to the aecium, 

 i.e., it is unable to propagate itself. The only haploid fructifications which 

 arise on the haploid mycelium, pycnospores, are generally not functional. 

 In the heteroecious forms, therefore, the change of host of the gameto- 

 phyte is obligatory; the gametophyte can only develop further if the 

 aeciospores are able to reach the alternating host. It is different with 

 the sporophytes. Here binucleate mycelium with urediniospores may 

 always arise from the urediniospores as long as climatic conditions or the 

 development of the host plant allows, so that an indefinite number of 

 generations of uredinial mycelium and urediniospores follow one another. 

 As the urediniospores, at germination, always infect the individuals of 

 the same species of host as that on which they have arisen, the sporophyte 

 is able to multiply indefinitely (as inOlpidium Viciae, by the zoosporangia). 

 Consequently in the heteroecious species, the change of host is only 

 facultative ; only if the sporophyte is forced by environment to form telio- 

 spores is it obligatory to change host, as the basidiospores can only 

 infect individuals of the gametophytic host. From the point of view 

 of change of nuclear phase, this innate unlimited repetition leads to an 

 increased duration of the sporophyte which subsequently predominates 

 over the gametophyte. 



Forms which correspond to this scheme of development, in which the 

 gametophyte forms pycnia and aecia and the sporophyte uredinio-, telio- 

 and basidiospores, are called "eu-" forms (with complete life cycles) 

 as Eu-puccinia or Eu-uromyces. In systematic literature, these spore 

 forms are indicated by the symbols 0, I, II, III, where indicates 

 pycnia which are often facultative in appearance, I aecia, II uredinia 

 and III telia. The basidia are not specially mentioned in this case as 

 they always appear at the germination of the teliospore. Of the cyto- 

 logically investigated forms, Cronartium ribicola, Puccinia graminis, 

 Phragmidium violaceum and Puccinia Violae belong to this type. 



Besides these Eu-forms, a great number of species are known in which 

 one or another of the spore forms is absent and the life cycle is "incom- 

 plete." These other types of cycle are given special names: the four 

 most important, -opsis, endo-, brachy- and micro- types, will be discussed 

 here. 



In the -opsis forms, e.g., in Gymnosporangium and in Puccinia Falcariae, 

 the urediniospores are lacking, in other species the pycnia also. The 

 life cycle corresponds entirely with that of the eu- forms, only the expan- 

 sion of the sporophyte, because of the repetition of urediniospores, is 

 absent. In some earlier mentioned species, as Puccinia Senecionis and 

 Uromyces Hedysari-obscuri, this gap is filled by the apogamous repetition 



