172 THE AMERICAN MONTHLY [August, 



5. Micropuccinia, Schroeter : Only teleutospores known ; germinat- 

 ing after rest. 



6. Leptopuccinia, Schroeter: Only teleutospores known ; germinat- 

 ing at once. 



In so far as they are co-extensive with Puccinia the other genera of 

 the order are classified upon the same basis. 



Hetercecism. — The life-history of the Heteropucciniae, so far as au- 

 thentically known, is by natural characteristics of development, conse- 

 quent upon an alternation of generations, divided into three distinct 

 periods, each one of which forms a complete fruiting stage, with spores 

 which, upon germination, develop directly into the vegetative tissues of 

 the fungus — i. e., into the mycelial filaments which penetrate the tis- 

 sues of the supporting plant. These stages, considered in the order of 

 their development from the teleutosporic sporidium, are known as 

 the ascidium, the uredo, and the teleutospore forms of the fungus, the 

 first two stages corresponding respectively to the genera yEcidium and 

 Uredo. These genera are still retained by systematists, and embrace 

 such ascidia and uredo forms as have not as yet been identified with a 

 teleutosporic form, which, when a full life-history has been determined, 

 takes precedence of the other spore forms, according to most authors, 

 giving the name to the species. 



Most species of the uredines complete their whole life-history upon one 

 host, each spore-form as it germinates being able to send the resulting 

 hyphae into the tissues of the host upon which the spores were first 

 formed, as in the Autopuccinia. Even if one or more of the spore-forms 

 are absent, as in the Leptopuccinia, we may be moderately sure that 

 the development is complete, for the sporidia which are abscised from 

 the promycelium of the germinating teleutospores immediately develop 

 the vegetative hyphae within the tissues of the same host which bore the 

 mother spores.* However, this is not the case in those species which 

 attack the grasses and other glumaceous plants. In every case only the 

 last two stages will be found upon these plants, while the aecidia, with 

 which are associated the spermogonia, when known, are always found 

 upon some non-glumaceous host. This is hetercecism as applied to 

 Uredineae. 



That such a change of host plants may occur during the development 

 of a single species is no longer questioned. Since the affirmation of 

 the same by De Bary in 1865, with regard to Puccinia graminis, the 

 common wheat rust, his results obtained by means of artificial cultures 

 have been confirmed time and again by experimental botanists until 

 the Heteropucciniae of which the life-history for at least one series of 

 hosts is accurately known to number over twenty species, f among which 

 are included many species most destructive to the various cereals and 

 grasses. The simple fact of the hetercecious nature of these parasites 

 is not now of so great interest to the mycologist in itself as the question 

 why it exists. Plowrightj suggests that these species are heteroecious 

 because the hosts upon which the uredo and teleutospores are devel- 

 oped, the grasses and sedges, possess silicious cuticles, which the 

 sporidia, perhaps, are not able to penetrate. Yet it may be said, in 



* De Bary, Morphology and Biology of the Fungi, p. 284. 



t See comparative table of aecidia and teleutosporic species in Plowright's " British Uredinea; and 

 Ustilagineae," p. 56. 

 *L. c.,p. 57. 



