MYCELIUM 313 



cia in Australia. The woody galls on various pines produced by 

 Cronarthnn qiiercinnn may be 2 or 3 times the diameter of the 

 stems on which they are formed. Some rusts cause the branch 

 axes to be foreshortened, resulting in the formation of witches' 

 brooms. Among such rusts are Gymnospormigmm nidiis-avis on 

 Juniperus, Ravenelia pygviea on tropical species of Euphorbia, 

 R. volkesnii on species of Acacia in Africa, Piiccinia caricis on 

 Urtica parviflora in central Asia, and Calyptospora goeppertiana 

 on \^accinium. 



Mycelium. The mycelium of rusts consists of branched hy- 

 phae that with a few notable exceptions are intercellular. Inti- 

 mate contact with invaded tissues is made by means of haustoria. 

 Thev are usually bulbous but sometimes variously branched and 

 knotted. The opinion that the haustoria may not penetrate the 

 protoplast was expressed by Colley (1918) from observations 

 on Cronarthnn ribicola and by Rice (1934), who studied Uro- 

 viyces caladii. In these species, and presumably in rusts gen- 

 erally, the haustoria are invaginations, and the ratio of their sur- 

 face area to host-cell volume establishes a nutritional balance that 

 does not result in the death of the invaded cells. Pady (1935) 

 has shown that the mycelium of the short-cycled form of Gym- 

 noconia interstitialis on Rubus is intracellular. Goplaiia mirab- 

 ilis on the foliage of Michelia velutina in Java is said to have a 

 superficial mycelium. 



The mycelium may be short-lived or perennial. Klebahn 

 (1904) lists 44 species in which the mycelium perennates. Among 

 fungi with perennating mycelium are such well-known species 

 as Cranarthmi ribicola, C. querciiimi, C. coleosporoides, attack- 

 ing conifers, Gyvmosporaiigiinn clavipes and G. iiidiis-avis on 

 Juniperus, Fhragmidiimi siibcorticimim on roses, Uromyces tri- 

 folii on clovers, Kitnkelia nitens on Rubus spp., Melampsora pini- 

 t or qua, Chrysomyxa abietis, and Endophylliim eiiphorbiae-syl- 

 vatici. 



Rust mycelia consists of two well-marked phases that alternate, 

 a monocaryotic phase and a dicaryotic phase. All too commonly 

 this alternation has been erroneously interpreted as alternation of 

 generations. Mature teliospores, promycelial cells, sporidia, the 

 mycelium arising from the germination of sporidia, and pycnio- 

 spores are monocaryotic. Aeciospore mother cells, aeciospores, 

 the mycelial cells resultant from germination of aeciospores, ure- 



