764 ECOLOGY 



Heteroecious and autoecious parasites. If a parasitic fungus has but a single 

 host in its life history, it is called autoecious, while if different stages occur habitually 

 on different hosts, it is called heteroecious. In the smuts there is an alternation of 

 saprophytic and parasitic phases (see p. 81), but in the heteroecious rusts, in addi- 

 tion to a similar alternation, the parasitic phase is partly on one host and partly on 

 another. Representative heteroecism is illustrated by the wheat rust, Puccinia 

 graminis, whose saprophytic phase occurs in spring, when the resting spores, known 

 as teleulospores (fig. 1127), germinate on the ground, and give rise to a mycelium, 

 which produces basidiospores (fig. 194). If any basidiospores, scattered by the 

 wind, chance to fall on a barberry leaf, a parasitic mycelium is developed, which 

 in turn produces spores known as aecidios pores (fig. 196). If these spores are carried 

 by wind to wheat leaves, they develop into a mycelium which in summer gives rise 

 to reddish spores, the uredospores (fig. 1125), which also germinate on wheat leaves. 

 Later in the season the same mycelium gives rise to black, thick-walled teleutospores 

 (figs. 1126, 1127), which fall to the ground and remain dormant until spring. An- 

 other well-known heteroecious fungus is Gymnosporangium, whose aecidial stage 

 occurs on Pyrus or on other Rosaceae, while the teleutospore stage occurs on Ju- 

 niperus, causing the galls known as cedar apples. 



One of the most remarkable features of the heteroecious rusts is their diverse 

 behavior. In some species of rusts certain stages are habitually lacking; for ex- 

 ample, the uredo stage may be absent, and sometimes the teleuto stage also, the life 

 history then consisting solely of aecidial and saprophytic stages. Again, the aecidial 

 stage may be absent, as in Puccinia Malvacearum, resulting in an alternation of 

 the saprophytic and the uredo-teleuto stages ; or both the aecidial and uredo stages 

 may be wanting, the basidiospores giving rise directly to a teleutospore mycelium. 

 Moreover, there are strictly autoecious forms, such as Puccinia Asparagi, in which 

 the aecidial stages and the uredo stages develop from the same mycelium. Still 

 more interesting are the variations within a single species. The wheat rust, for 

 example, thrives in regions where the barberry does not exist, partly because the 

 uredo mycelium even in very cold climates is able to hibernate in winter wheat, thus 

 eliminating the necessity of either saprophytic or aecidial stages, and partly because 

 uredospores that have survived the winter may infect young wheat the following 

 spring; even the teleutospores or basidiospores may directly infect young wheat 

 in certain instances. In tropical climates Uromyces Fabae, a parasite of Vicia 

 Faba, spreads solely by uredospores. Thus heteroecism is seen to be in part facul- 

 tative; it is obligate, however, in certain cases, as in Coleosporium Melampyri, 

 whose host is an annual. 



Sometimes it has been thought that the ease with which rusts make new infec- 

 tions cannot be explained merely by their remarkable variability. Consequently 

 it has been suggested that under certain conditions the protoplasm of the fungus 

 and the host may merge into a common mycoplasm, and in this invisible or imper- 

 fectly evident form, the fungus is thought to exist in the seed. When the seed 

 germinates, the fungus is supposed once more to become differentiated into an 

 obvious mycelium. This strange theory of the mycoplasm was formulated to 

 account for cases of parasitism, in which all evident external sources of infection 

 are lacking. Such a theory, appearing to defy the possibility of experimental 

 analysis, like the theories of vitalism and of adaptation, should be resorted to, if 



