SUBCLASS TELIOSPOREAE 381 



meiotic divisions then take place in the promycehum instead of the body 

 of the teliospore. In some cases the nuclei divide once or twice more 

 resulting in the production of 8 or 16 or even more nuclei. The promyce- 

 lium may remain nonseptate but more often becomes transversely septate 

 into four cells. From each of these cells is produced a sessile or stalked 

 sporidium or several such sporidia. From the nonseptate promycelium 4 

 to 16 or more sporidia bud out at the apex. If the teliospore is deeply 

 buried in the tissues of the host or among other teliospores the pro- 

 mycelium at first is a slender hypha, its emergent portion developing the 

 typical structure. 



The sporidia are borne in the Uredinales and in Family Tilletiaceae 

 in the Ustilaginales at the tips of pointed sterigmata from which they are 

 flung violently at maturity. Their position is slightly asymmetrical at 

 the tip as is the case in the Heterobasidiae and in the Hymenomyceteae. 



Many writers (e.g., Gaumann, 1926, Arthur, 1929, etc.) apply 

 the names basidium and basidiospore, respectively, to the struc- 

 tures here called by their older names promycelium and sporidium. The 

 latter is doubtless homologous to the basidiospore but the term basidium 

 must include both the teliospore, in which the nuclear fusion occurs, and 

 the outgrowth from it, the promycelium, in which the meiotic nuclear 

 division usually occurs and upon which the sporidia are borne. More 

 correctly, as will be seen further on in the next chapter, the teliospore is 

 probably homologous to the hypobasidium and the promycelium to the 

 epibasidium of some of the Auriculariales. 



Asexual reproduction is known in both the Uredinales and the 

 Ustilaginales. In the latter it occurs by means of colorless, often spindle- 

 shaped or sickle-shaped conidia which are uninucleate or binucleate. The 

 latter arise only from dicaryon mycelium but the uninucleate conidia 

 may arise from either monocaryon or dicaryon mycelium. They arise on 

 very short conidiophores, or rather sterigmata, from the sides of the 

 mycelial cells. They are usually in the true sense of the words "repeating 

 spores" for they produce the same type of mycelium as that from which 

 they arose, except in the case of uninucleate conidia from dicaryon 

 mycelium. These give rise to monocaryon mycelium. These conidia are 

 mostly produced on the saprophytic mycelium, but in some species are 

 developed on sterigmata which emerge through the epidermis of the living 

 host. In the Uredinales the urediospores (uredospores) are dicaryon 

 repeating spores and are, therefore, true conidia. There are no mono- 

 caryon repeating spores in the Rusts. The aeciospores (aecidiospores) are 

 the result of a sexual fusion involving cells but not the nuclei. They bridge 

 over the step from the monocaryon phase to the dicaryon phase. They 

 are not strictly homologous to ordinary conidia but yet show great 

 resemblance to the urediospores. 



