154 DISEASES OF TREES 



Besides the aecidium, a kind of gonidium is almost always 

 formed, which, being designed to carry the fungus over from one 

 year to another, has great capacity for retaining its power of 

 germinating. This is known as a resting-spore or teleutospore, 

 and, instead of directly forming a mycelial filament, it first pro- 

 duces a promycelium, on which a number of small cells, called 

 sporidia, develop, and it is these which produce the disease by 

 infecting new host-plants. The teleutospores are unable to 

 produce infection, because they are usually in such intimate 

 contact with the substance of the host-plant that their distribu- 

 tion by the air is almost entirely precluded. The mycelium that 

 is developed from the sporidia again produces spermogonia, and 

 after fertilization sporocarps (aecidia). Thus an alternation 

 of generations between the two forms of aecidia and teleutospores 

 is presented, which, however, in the case of many rust-fungi, is 

 further complicated by the fact that a form bearing teleutospores 

 is not directly produced by the germinating aecidiospores, but 

 that numerous generations of gonidia of another kind namely, 

 uredospores often originate. These at once germinate, without 

 forming a promycelium, and reproduce the form bearing uredo- 

 spores. During summer they serve for the rapid distribution of 

 the fungus, till the teleutospores are produced by the mycelium, 

 as usually happens in autumn. The cycle of development of 

 many rust-fungi is interesting, from the fact that not only the 

 uredo form but also the aecidium form may possess a facultative 

 character ; that is to say, these forms may develop only under 

 certain favourable circumstances, and in the absence of such 

 conditions they may be entirely omitted without the existence 

 of the parasite being thereby imperilled. 



The generation which forms the aecidia, and that which pro- 

 duces the teleutospores, are either to be found on the same plant 

 (autoecious parasites), or, with the alternation of generation, 

 there also occurs a change of the species of host-plant (heter- 

 cecious parasites). In the case of the hetercecious rust-fungi the 

 discovery of the related forms which belong to one and the same 

 species of fungus naturally presents great difficulties. This is 

 sufficient to explain why we are not at present acquainted with 

 the aecidia of many teleuto forms, and, on the other hand, do not 

 yet know to which teleuto forms many aecidium forms belong. 



