ORDER UREDINALES (tHE RUSTs) 399 



main uninucleate and produce a two-celled promycelium. The spermo- 

 gonia in this race are never more than rudimentary and no receptive 

 hyphae are formed. (Fig. 131.) 



A third type of life cycle is the one usually designated as 0, II, III, 

 i.e., a cycle in which the typical aecial structure is lacking. The spermo- 

 gonia are succeeded by sori containing spores exactly resembling typical 

 urediospores. In the same way as are produced the dicaryon basal cells of 

 the chains of aeciospores Christman (1907) showed that there are pro- 

 duced dicaryon cells giving rise to urediospore-like structures. The latter 

 give rise to dicaryon mycelium from which may arise later another series 

 of urediospores. The usual interpretation of this phenomenon is that this 

 is really a macrocyclic rust of the formula 0, I, II, III in which the aecio- 

 spores are not produced in chains but singly on stalks, like urediospores. 

 The primary (first produced) urediospores are therefore modified aecio- 

 spores while the secondary ones are true urediospores. 



The aecium may be cup-shaped (cupulate) with a well-developed pe- 

 ridium or it may be very tall so as to make a horn-like (cornute) structure. 

 The peridium may be lacking so that the aecium is diffuse. Other forms 

 are known, the most curious of which is the "hyphoid" aecium of Dasy- 

 spora foveolata B. & C. (D. gregaria (Kze.) Henn.) in which a branching 

 dicaryon mycelium emerges through various stomatal openings, forming 

 a colorless mass of hyphae. These are terminated by single, not catenulate, 

 aeciospores which drop off while the hypha elongates sympodially and 

 produces another spore and so on. The cytology of this type of aecium 

 needs careful investigation to determine where the diploidization occurs. 

 Sydow (1925) interprets these as urediospores but Arthur and co-authors 

 (1929) consider them to be aeciospores. (Fig. 132.) 



The uredium may be merely a cluster of stalked urediospores bursting 

 through the epidermis of the host or it may be surrounded by paraphyses. 

 In Cronartium and Puccmiastrum and closely related genera the uredium 

 possesses a true peridium. In some genera, e.g., Coleosporium the uredio- 

 spores, like the aeciospores, are produced in chains but they arise from a 

 dicaryon mycelium and so differ from the latter. In a few rusts the ured- 

 ium is cupulate and resembles the aecium but spermogonia are lacking. 

 They are sometimes called secondary aecia. Cummins (1937) reported 

 that the uredium of Prospodium is a salver-shaped structure whose slender 

 stalk about three cells thick emerges through a stoma and spreads out as a 

 flat plate with upright fringe-like marginal paraphyses. On the flat surface 

 of the salver arise the stalked urediospores. In some species of the genus 

 the telial sorus is quite similar. (Fig. 133.) 



The telium is the most variable structure in the order. Properly speak- 

 ing a teliospore is a single cell, binucleate at first but becoming uninucleate 

 by the fusion of the two nuclei and giving rise immediately or after a delay 



