A Working Hypothesis on the Origin of Rusts, with Special Reference to the Phenomenon of Heteroecism

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Abstract
Heteroecism and ontogeny in the rusts are discussed from the following standpoints: alternation of generations; fixity of organs; acquisition of parasitism; length of cycle in the phylo-genetic trend[long dash](1) relative numerical importance of different life-cycles, (2) geographic distribution of short-and long-cycle species, (3) correlations between short-and long-cycle species, (4) short and long cycles in relation to climate; morphology of the different sori; homologies with independent plants; host relationships; different types of correlations; evolutionary progress oi host plants; antithesis between gametophyte and sporophyte. It is postulated that Uredinopsis represents most nearly the primitive type of rust which was long-cycle and heteroecious. From the homologies existing between rusts like Uredinopsis and certain dimorphic red algae, it is suggested that the Uredinales originated from the Florideae through an endophytic development in which the gametophyte and sporophyte adapted themselves independently on different hosts, such as Abietineae and ferns. Antoecism may have resulted from the 2 generations becoming adapted to the same host. The presence of correlations between long- and short-cycle rusts, together with the effects of parasitism and other phenomena, is evidence that the latter are degenerate forms of the former.

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