318 THE BASIDIOMYCETES 



(trichogynes) among rusts. Furthermore they are produced 

 from the same haploid mvceHum as bears spermatia. Self-fertili- 

 zation does not occur, but, as would be anticipated, the thalli are 

 cross-fertile and cross-compatible. Heterothallism is now defi- 

 nitely known in Puccinia gramwiSy P. helianthi, P. tritichia, P. 

 coronata, P. sorghi, P. pr'mgsheimiana, Melavipsora lini, Crojiar- 

 tium ribicola, Uromyces appendiciilatiis, U. fabae, U. trifolii- 

 hybridi, U. vignae, and Gyvmocoma biterstitialis. It must by 

 no means be inferred that all rusts are heterothallic. Brown 

 (1939) offers evidence that a single sporidium of Puccinia coro- 

 nata-elaeagm, P. grindeliae, and P. xanthi gives rise spontaneously 

 to binucleate mycelium that is homothallic. 



The receptive hyphae are composed of uninucleate cells, ac- 

 cording to Andrus (1931), Pierson (1933), and Allen (1934a), 

 and they extend out from uninucleate basal cells of the aecial 

 primordium. Craigie (1931), using Puccinia helianthi, Allen 

 (1934a), using F. sorghi, and Buller (1938), using P. graminis, 

 have shown that receptive hyphae extend from the ostiole of the 

 spermogonium. The spermatium has been shown to fuse with 

 the receptive hvpha (paraphysis or trichogyne) and to pass down 

 through the hypha to the basal cell {&<^^ cell). There the two 

 nuclei associate in what is regarded as the aeciospore mother cell. 

 Here they divide conjugately and repeatedly, and the mother 

 cell gives rise to a series of dicaryotic aeciospores. Allen (1932) 

 has shown in Puccinia triticina that hyphae other than those near 

 the spermogonial orifice may serve as receptors. Instead recep- 

 tive hvphae may emerge from stomatal openings or may even 

 penetrate to the surface between epidermal cells. 



Diploidization may be accomplished otherwise, as is indicated 

 by Brown (1935), who found evidence that hyphal fusions may 

 occur in P. helianthi w^hen two haploid pustules of opposite sex 

 phases coalesce. In this case the spermatia did not function, but 

 there is reason to believe that the dicaryotic condition in rusts is 

 quite generally achieved through the agency of spermatia. 



Aecia. The aecia are of four form types: aecidium, caeoma, 

 roestelia, and peridermium. These terms are sometimes used as 

 generic names to designate the aecial stages. If the chains of 

 aeciospores are formed compactly within a membrane or pe- 

 ridium that, on opening, is cup-like, the structure is spoken of as 

 an aecidium. If the peridium is lacking and the chains of aecio- 



