HETEROECISM 331 



act as a bridge supports this argument. For example, the aecio- 

 spores of Cronartium asclepiadmm, formed on Piims syhestris, 

 infect and produce teha on members of 4 families of flowering 

 plants: Asclepiadaceae, Verbenaceae, Scrophulariaceae, and Ra- 

 nunculaceae. Again, the aeciospores of Melampsorella caryo- 

 phyllaceariim initiate infection on species among distinct genera 

 of Caryophyllaceae. On the other hand, quite the opposite con- 

 dition may result, as it does with 7 species of heteroecious Puc- 

 cinia that bear aeciospores on members of 3 or 4 families of 

 monocots but that all produce teliospores on Phalaris arimdi- 

 nacea. 



The opinion of Jackson (1931) and nearly all present-day stu- 

 dents of rusts is not only that the autoecious species have been 

 developed from heteroecious species but also that all species with 

 a reduced cycle typify present-day trends among rusts. These 

 short-cycled species are therefore derived from autoecious 

 species. Olive (1911) and a group of other workers, such as 

 Klebahn, Magnus, and Dietel, regarded lepto and micro forms as 

 the ancestors of heteroecious rusts. They would argue that Piic- 

 cinia mesneriana, one of the crown rusts, whose telia are produced 

 on Rhamnus, is the progenitor of Fiiccinia coronata, which re- 

 quires Rhamnus as its aecial host and Avena as its uredinial and 

 telial host. Autoecism is therefore primitive, according to this 

 group of students, and heteroecism has arisen as an adaptation. 



Evidence that the long-cycled forms are ancestral and that 

 they may give rise to forms with simpler cycles is derived from 

 studies of Fuccinia podophylli. This species hibernates as telio- 

 spores, and the sporidia may initiate infections that early give rise 

 to teliospores on the bud scales and stems. Shortly thereafter 

 aecia appear on the leaf blades, and they too arise from the same 

 primary source [Whetzel, Jackson, and Mains (1925)]. Pycnia 

 accompany these aecia but are usually absent from the early 

 crop of teliospores. In late summer teliospores are produced on 

 mycelium from infection by aeciospores. Fuccinia podophylli 

 is therefore not fixed but plastic in its cycle of development. 



It would appear more than a coincidence that species of Ure- 

 dinopsis, Milesia, and Hyalospora have their uredinial and telial 

 stages on ferns and their other stages on firs, that nearly all species 

 of Gymnosporangium bear their telia on Juniperus or closely 



