148 ] The Classification of Lower Organisms 



The first-formed reproductive structures of the dikaryote mycelium on the telial 

 host are usually uredospores, which remain binucleate and have the function of 

 spreading the infection of the telial host. 



Teliospores may be compacted into palisade-like masses which break through the 

 epidermis of the host; the masses may be gelatinous and yellow, like fruits of 

 Auriculariacea. In other genera, the teliospores are gathered into hard, microscopic- 

 ally stout columns, and in yet others they break through the epidermis in masses not 

 compacted, each teliospore on a separate stalk. The teliospores of Phragynidium are 

 chains of several probasidia; those of the many species of Pucciriia are chains reduced 

 to two probasidia; those of Ravcnelia are globular clusters of probasidia. Almost 

 always, the teliospores are thick-walled; outside of the tropics, they have the function 

 of overwintering. Each probasidium contains two nuclei. These unite as a preliminary 

 to germination: this was first observed by Sappin-Troufi^y (in Dangeard and Sappin- 

 Troufi'y, 1893). Thereafter the probasidium gives rise to the four-celled promycelium. 



The life cycle thus described is not perfectly stable. Aeciospores, uredospores, and 

 young teliospores are alike dikaryote, and are genetically identical. Spores of the 

 structure and behavior of any of these types may be produced by processes which 

 normally lead to another. Thus in Puccinia Malvaccaruvi, the hollyhock rust, syng- 

 amy leads directly to the production of teliospores on the host of the haploid mycel- 

 ium; spermagonia, aecia, and uredosori are not produced. 



Four families of rusts may be recognized (various authorities make fewer or more). 

 There are about five thousand species. 



Family 1. Melampsoracea [Melampsoraceae] Dietel in Engler and Prantl Nat. 

 Pflanzenfam. I Teil, Abt. 1**: 38 (1900). Teliospores forming a single compact 

 layer and germinating by producing promycelia. The aecial stages are mostly on 

 conifers. Some have telial stages on ferns, and FauU (1929) regards these as most 

 primitive; others attack a variety of flowering plants. 



Family 2. Coleosporiacea [Coleosporiaceae] Auctt. The teliospores themselves 

 becoming basidia by transverse division. In some examples, as Gallowaya, they are 

 thin-walled. 



Family 3. Cronartiacea [Cronartiaceae] Auctt. The teliospores compacted into 

 columns. Cronartium, with aecial stages on pines; C. ribicola, the important white 

 pine blister rust, its telial stage on gooseberries and currants. 



Family 4. Uredinacea [Uredinaceae] Cohn in Hedwigia 11: 17 (1872). Family 

 Pucciniaceae Dietel op. cit. 48. The bulk of the rusts, producing teliospores on indi- 

 vidual stalks. Hemileia vastatrix, the coffee rust; Phragmidium spp., autoecious (at- 

 tacking a single host) on Rosaceae; Gymnosporangium, the aecial stage on junipers, 

 the telial (with no uredospores) on plants of the apple tribe; Puccinia, a great num- 

 ber of species. The races which attack barberry and grasses are all called Puccinia 

 graminis; but there are morphologically distinguishable strains on wheat, rye, oats, 

 timothy, Agrostis, and blue grass. Leading an active sexual life and capable of muta- 

 tion, these strains are subdivisible into large numbers of races distinguished by capa- 

 city to attack different races of hosts. Given a specimen of rust on wheat, one deter- 

 mines by trial upon seedlings of ten varieties of wheat to which of 189 numbered races 

 it belongs. The races occur characteristically in different wheat-growing areas. If one 

 breeds wheat for resistance to rust, there is good probability of success against the 

 races occurring locally; but some other race is likely to move into the area (Stakman, 

 1947). 



