388 



It is evidently a right conception, in view of the foregoing statement, 

 to regard Puccinia graminis (a better name is Dicccoma poculiforme) as 

 a representative of the highest development of rusts. But to regard it as 

 typical of all rusts, or even of all rusts having recia, is clearly asking too 

 much of an illustration, and likely to involve grave misconceptions of 

 structure and relative values. If the most essential features of the rusts 

 were to be illustrated by the smallest permissible number of examples of 

 common and well known species, I should spler-t Polythelis Thalictri (Puc- 

 cinia TTialictri) for the forms without aecia, Kuelmeola aibida (often called 

 Chrysomyxa aibida) for the forms with secia and isomorphic repeating 

 spores, and Dicaioma poculiforme (Puccinia graminis) for the highly de- 

 veloped forms with a?cia and heteromorphic repeating spores. 



A wrong conception, which is doing much harm to the taxonomic study 

 of the rusts, is the view that jeciospores and urediniospores are of the 

 nature of conidia, that is, asexual spores, comparable to the conidia so 

 abundantly produced by many ascomycetous fungi. Cytological studies 

 show, however, that in the rusts the only truly asexual spores, other than 

 the basidiospores, are the pycniospores, and to these only can the term 

 conidia be applied with approximate accuracy. The sexual process begins 

 by the fusion of uninucleated hyphal cells, which immediately, or almost 

 immediately, develop some kind of binucleated spore-structure. If only 

 one kind of binucleated spore is produced by the species, it is properly 

 called a teliospore. Such a teliospore has two nuclei in each cell, derived 

 by a short succession of divisions from the two nuclei of the fusing cells. 

 These two spore nuclei fuse into one nucleus prior to germination of the 

 teliospore, thus completing the sexual process. If more than one kind of 

 binucleated spore is produced, the initial kind may be called an aeciospore, 

 whatever the morphological structure in which it is formed. It has 

 arisen as the consequence of sexual cell fusion, just as in the preceding 

 case, and has the physiological character of greatly stimulated growth 

 associated with sexuality. This initial reciospore gives rise to a binucle- 

 ated mycelium, which in turn generally produces binucleated repeating 

 spores of the same or of a different form, and so on, until finally a telio- 

 spore is produced in which nuclear fusion takes place, as in the first 

 instance mentioned. The sexual process in this class of rusts extends 

 from the cell fusion at the base of the aecia through all the succession of 

 hyphal cells and repeating spores to the fusion of nuclei in the mature 

 teliospore. 



