ORDER UREDINALES (tHE RUSTS) 



383 



Fig. 126. Monocaryon and dicaryon mycelium and haustoria in Subclass Telio- 

 sporeae, Order Uredinales. (A) Cronartium ribicola Fischer, monocaryon phase in 

 tissues of Pinus strobus L. ; intercellular hyphal cells and haustoria uninucleate. (B) 

 Phragmidium rubi (Pers.) Wint., dicaryon phase in leaf of Rubus sp.; intercellular 

 hyphal cells and haustoria binucleate. (A, courtesy, Colley: /. Agr. Research, 15(12): 

 619-660. B, after Sappin-Trouffy : Le Botaniste, 5:59-244.) 



of miles in some cases). In general in this order the teliospores remain 

 attached in the sorus until after the formation and discharge of the 

 sporidia. However, Pady (1948) has shown that in Puccinia tumidipes 

 Peck the very much swollen pedicel will absorb water and rupture, throw- 

 ing the teliospore some distance (2 to 4 mm.). It is probable that the same 

 phenomenon occurs in other species with enlarged pedicels, such as some 

 species of Phragmidium and Gyinnospor-angium. 



The mycelium occurs in two well-marked alternating phases, the 

 monocaryon and the dicaryon phase. Clamp connections have been 

 reported by Voss (1903) on the mycelium of the aecial phase but are 

 apparently rare, or at least not readily demonstrated in thin sections. 

 Their presence was confirmed by Wang and Martens (1939) in close 

 proximity to the aecium of Puccinia coronata Cda. on Rhamnus frangula 

 L. They were demonstrated by maceration of the tissues, not by sections. 

 Haustoria are frequent, sometimes small and spherical, sometimes large 

 and branched, and often partially surrounding the nucleus of the host 

 cell. They are uninucleate or binucleate according to the type of mycelium 

 producing them. Colley (1918) showed for the White Pine Blister Rust 

 {Cronartium ribicola Fischer) that the haustorium does not truly pene- 

 trate the protoplast of the host cell but causes invagination. Miss Rice 

 (1934) demonstrated this also for Uromyces caladii (Schw.) Farl. Appar- 

 ently the host and parasite stand in a very perfect balance for a very 

 considerable time, especially in the case of rusts that are well adapted 

 to the host. The host cells are not killed outright in such cases. In some 

 of the rusts that are not well adapted to their host Stakman (1914) 

 demonstrated that the host cells are killed immediately around the site of 



