582 COMPARATIVE MORPHOLOGY OF FUNGI 



in the basidium and the basidiospores again become binucleate (Fig. 

 393, 6). Similarly, it is assumed that in Puccinia Podophylli the telio- 

 spore-forming apogamous aecia may arise on the perennial binucleate 

 mycelium (Olive, 1913). In Endophyllum Valerianae-tuberosae (Maire, 

 1900), in which caryogamy is sometimes absent, one nucleus in the 

 aeciospore degenerates, leaving the basidiospore uninucleate. 



Since we have discussed individually each of the five spore forms of 

 the rusts, we will now consider the form and manner in which the life 

 cycles of the different species are related. As representative of the type 

 in which all forms appear, may be cited the beet rust, Uromyces Betas. 

 In it there appear in spring from the basidiospores, uninucleate mycelia 

 which form pycnia with pycnospores and aecia with aeciospores. At the 

 base of the aecial chains, plasmogamy occurs. The aeciospores are 

 dispersed to other beet leaves, are capable of immediate germination and 

 develop to new binucleate mycelia on which appear uredinia and uredinio- 

 spores, able to germinate immediately and to infect new leaves, where 

 again they may form binucleate mycelia on which uredinia appear. 

 Toward autumn, telia with teliospores arise on all these binucleate mycelia. 

 Caryogamy occurs in the teliospores which winter on the fallen leaves 

 and in the spring germinate meiotically with basidia and basidiospores. 

 The basidiospores again reach young beet leaves and develop there to 

 uninucleate mycelia. 



The life cycle which takes place in the limits of a growing period, at 

 least under European conditions, may be expressed in the following 

 scheme: 



p I c R 



yPycnium-wPycniospores \ JL'redimospores 



Uninucieate^Aecium->Sexualcelle-*Aeciospores-»BinucIeate-^TeIio8pores-^Basidia-*Ba8idiQ3pores. 



mycelium mycelium 



Diagram XXXIII. 



In this diagram it is noteworthy that haplont and diplont, as in 

 Olpidium, possess individual thalli. In the life cycle of Uromyces Betae 

 an alternation of generations proceeds with change of nuclear phase. 

 The gametophyte consists of the uninucleate mycelium which arises 

 from basidiospores and forms pycnia and aecia. The sporophyte consists 

 of binucleate mycelium arising from aeciospores and forming first uredin- 

 iospores and then teliospores. As both gametophyte and sporophyte 

 in Uromyces Betae live on the same host, this alternation of generations 

 can only be discerned cytologically. In the heteroecious forms, it coin- 

 cides with alternation of hosts; thus, as has been shown above, Cronartium 

 thrives in the haploid portion on Pinus and in the diploid portion on 

 Ribes. Gametophyte and sporophyte here differ physiologically and are 

 specialized on hosts systematically far apart. 



