1890.] PUBLIC DOCUMENT — No. 33. 207 



tissues of their hosts. The mycelium is largely used up in the 

 formation of spores, so that, at maturity, little is to be found 

 but a dark-brown or black powdery mass of spores. In 

 most cases these lattea* can germinate at once under certain 

 conditions ; but they may live for a very long time ready to 

 germinate when favorable conditions occur. The spores of 

 some smuts seem to be true resting spores ; and those of 

 many other species approach that condition, in that they 

 germinate much more readily after a period of rest than 

 when just mature. The smuts of corn, of wheat and other 

 grains, and of the onion, are only too well known. 



7. liusls ( Uredinece) are especially interesting for their 

 striking and remarkable pleomorphism, already referred to. 

 They are very common parasites of flowering plants, and the 

 typical species produce three chief spore forms. Individual 

 variations within the group make it difficult to give a general 

 account, but the following will apply to most of the rusts. 

 Early in the season the fungus appears in its first or cluster- 

 cu}) stage, described above, and shown in the yellow patches 

 so common on barberry leaves in June. The spores of this 

 form produce fresh mycelia, which give rise to the second, 

 and later to the third, spore form. These second and third 

 forms, are, as has been already stated, the red and hlach 

 rusts, respectively. This is the typically complete condi- 

 tion, but in very many cases one or even two of the forms 

 are unknown. The spores of the dusfer-cuj) and red-rust 

 forms are summer spores, while those of the black-rust are 

 usually resting spores, though not alwa^^s so. 



Frequently the various forms of a rust fungus follow each 

 other on the same host plant ; but the difficulty of a com- 

 plete knowledge of many of them is further complicated by 

 the fact that the cluster-cup form occurs on one host, and the 

 other two on a widely different one. For example, the 

 cluster-cup of the barberry is the first stage of the fungus 

 whose second and third stao-es are the red and black rusts of 

 wheat and various other grains and grasses, as has been 

 shown by careful and repeated cultures. This form of pleo- 

 morphism, in which the different spore forms of a parasitic 

 fungus occur on different hosts, is known as hetercecism. 



