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PLANT MORPHOLOGY 



fuse, thus establishing the diploid condition. The fusion nucleus divides 

 reductionally in the young basidium and four cells are formed by the 

 appearance of transverse walls, thus separating the four haploid nuclei. 

 When a basidiospore is budded off, two nuclei are formed, one of which 

 passes into the spore while the other remains in the basidium. The 

 latter may divide again, if another spore is budded off, and this may be 

 repeated many times. These haploid basidiospores produce on the young 

 corn plant mycelia of limited extent and with uninucleate cells. When 

 two mycelia of opposite sex come together within the host, a union of 

 cells takes place without a fusion of nuclei. The binucleate cells formed 

 in this way give rise to a mycelium that spreads throughout the host, 

 eventually producing chlamydospores. 



2. Uredinales 



The Uredinales, or rusts, are destructive parasites. They attack a 

 great variety of vascular plants, including ferns, conifers, and angio- 

 sperms, being especially common on grasses. The mycelium lives in the 

 intercellular spaces, particularly of the leaves. There are about 3,000 

 species of rusts, the most important genera being Puccinia, Uromyces, 



Fig. 117. Puccinia graminis. Section through a uredinium on a leaf sheath of wheat, 

 showing uredospores in various stages of development, X200. 



Gymnosporangium, Phragmidium, Cronartium, Coleosporium, and Metkrnp- 

 sora. The largest genus, Puccinia, has about 700 species. 



Puccinia. The common wheat rust {Puccinia graminis) is the best- 

 known member of the order. Its life history is very complicated, involv- 

 ing two different hosts and several kinds of mycelia and spores, all with 

 a definite relation to one another. 



The mycelium that lives on the wheat is an internal parasite, extend- 



