DEVELOPMENTAL CYCLE OF PUCCINIA GRAMINIS 311 



produced on the interior of pycnia and are extruded to the leaf 

 surface. 



While the pycnia are forming, other hyphal aggregates, the 

 initials of the aecia (cluster-cups), are being formed at points 

 more deeply embedded within the leaf tissues. Special hyphae 

 that protrude in tufts from the orifices of the pycnia are con- 

 nected with the young aecia beneath. These protruding hyphae 

 function as trichogynes. Evidence for such functioning rests in 

 part upon the fact that all the cells composing the aecia are uni- 

 nucleate, except the basal cells that initiate aeciospores (cluster- 

 cup spores). The aeciospore initials or aeciospore mother cells, 

 however, are binucleate (dicaryotic). By successive divisions of 

 the aeciospore mother cells chains of aeciospores are produced. 

 Pressure exerted on overlying leaf tissues by the developing 

 aeciospores causes the aecia to open, whereupon they have the 

 appearance of cups. The aeciospores, thus liberated, are dis- 

 tributed by winds. Those that lodge on a suitable host (wheat) 

 may germinate, and the germ tube enters the host tissues through 

 a stoma. An intercellular mycelium of dicaryotic cells will give 

 rise within a few weeks thereafter to uredinia (red-rust-stage 

 pustules), containing stalked, rust-colored, eUipsoidal, 1 -celled 

 but binucleate urediniospores. Urediniospores are air-borne, are 

 capable of immediate germination on wheat, and thus serve to 

 spread and increase the red-rust stage. The mycelium arising 

 from the germination of urediniospores is dicaryotic throughout. 



The same mycelium that gave rise to urediniospores eventually 

 produces 2-celled teliospores. Each young tehospore cell is bi- 

 nucleate, but before germination, during the succeeding spring, 

 the 2 nuclei fuse; thus the fusion nucleus becomes diploid. By 2 

 successive nuclear divisions that occur during the formation of 

 the probasidium, 4 haploid nuclei are again formed. These hap- 

 loid nuclei migrate singly into the sporidia, and the sporidia are 

 thus ready to start again the developmental cycle. 



The outstanding features of rusts are ( 1 ) their polymorphism, 

 as expressed by the production of 5 spore forms in the complete 

 life cycle; and (2) the necessity, for many species, of 2 unre- 

 lated hosts for the completion of their developmental cycle. In 

 the complete cycle of rusts there are teliospores, basidiospores or 

 sporidia, pycniospores (spermatia), aeciospores, and uredinio- 

 spores, formed in sequence. In some species 1 or more stages 



