586 COMPARATIVE MORPHOLOGY OF FUNGI 



isogamously by the dissolution of the separating septum. It leads to the 

 formation of a dicaryon which is adapted to multiple conjugate divisions. 



2. In all forms the gonotocont (which characterizes the Uredinales 

 as Basidiomycetes) is developed as a basidium which throughout the 

 whole order has a stereotyped form (except for variation in length of 

 sterigmata). In contrast to this fixity in form and function, it is very 

 variable in place of occurrence. It can (in the Coleosporiaceae and 

 Ochropsora) be formed directly on the mycelium; or it can (in the other 

 three groups) germinate from the most bizarre, encysted or thin-walled 

 teliospores; or it can develop from aeciospores which have arisen either 

 in aecidium or caeoma. Throughout all these variations, it remains 

 unaltered. 



3. The sequence of the five spore forms is controlled by internal 

 factors; it is irreversible. In the life cycle, one spore form or another 

 may be degenerate or disappear entirely but there is no species known, 

 parthenogenetic or apogamous, in which the sequence is changed. This 

 innate regularity is based in part on the change in nuclear phase, in part 

 on the great age of the Uredinales. 



The only stable moment in the life cycle of the Uredinales depends on 

 the fact that meiosis takes place in the same organ; on the other hand, the 

 place of origin of the basidium and the place of the sexual act are variable. 

 Furthermore, the gonotoconts are fixed in form (even to the basidium). 

 The haploconidia (basidiospores and pycnospores) and the diploconidia 

 (aeciospores and urediniospores) are fixed also. The zeugites (telio- 

 spores), on the other hand, are plastic. 



Up to the present, in as far as we have discussed the bare fact of the 

 ontogeny and morphology, the Uredinales agree with each other; there 

 is, however, a complete anarchy in regard to the significance of all these 

 forms and types of cycles. How have they arisen and how are they 

 related? The answer to this question is difficult, for the Uredinales 

 belong to the oldest known fungi and they have survived to our time, 

 isolated as "living fossils." There exist today some very primitive 

 genera but they are, for the most part, exotic and hence have not been 

 investigated cytologically. Thus this answer cannot be given without 

 considering at the same time the phylogeny of the Uredinales, their 

 systematic division and the origin of their biological specialization. 



Concerning the phylogeny of the rusts there are two concepts, that of 

 Bary (1884) which derives the Uredinales from the Ascomycetes and that 

 of Brefeld (1889) which connects them to the Zygomycetes. Many 

 mycologists today, as Blackman (1904), Lotsy (1907), Dittschlag (1910), 

 Kursanov (1910, 1922), Maire (1911), Fromme (1912), Mme. Moreau 

 (1914), Gwynne-Vaughan (1922), Lindfors (1924) and, in part, Killian 

 (1920) follow Bary's opinion; they consider the Uredinales as the most 

 primitive Basidiomycetes and place them at the beginning of this class. 



