UREDINALES 



581 



Under unfavorable conditions, as under water or in too low oxygen 

 pressure, the germ tube of many species, as Puccinia Malvacearum (Kle- 

 bahn, 1914) and P. graminis (Jaczewski, 1910), as also the aeciospores 

 with basidial germination as in Endophyllum Sempervivi (Nypels, 1897), 

 develops to long, slender, branched hyphae which form basidiospores 

 only when they reach the surface. In still other forms, as Gymnosporan- 

 gium clavariaeforme (Blackman, 1904) and occasionally also Puccinia 

 Malvacearum (Fig. 389, 10 to 13), the four basidial cells round up and fall 

 apart at the least shock; they germinate, according to the environment, 

 with a germ tube or a conidium (basidiospore). Whether these 



Fig. 393.- 



- Endophyllum Euphorbiae-silvaticae. Germination of apogamous aeciospores. 

 (X 665; after Moreau, 1919.) 



phenomena are abnormalities or have a biological meaning, is still 

 questionable. 



In Puccinia Helianthi, the mycelia from germinating basidiospores 

 are heterothallic. If they fail to anastomose with other mycelia of the 

 opposite sex, they produce only pycnia, while if a union of + and — 

 mycelia occurs, aecia are produced. Occasionally small aecia of uninu- 

 cleate aeciospores are produced by the mycelium from a single spore 

 (Craigie, 1927). 



In the apogamous forms (p. 569 ), e.g., Endophyllum Euphorbiae- 

 silvaticae (Moreau, 1919), the two nuclei of the aeciospore migrate into 

 the basidium, behave there as the two daughter nuclei of the primary 

 basidial nucleus, become separated by a septum (Fig. 393, 3) and develop 

 normally. The binucleate condition appears again, as in normal develop- 

 ment, at the base of the aecium or as a result of previous nuclear division 



