TUBERALES 359 



Various species of Tuber are used as food. 



While Tuber continues the Hydnotrya-Pachyphloeus series in a vertical 

 direction, a side line, the Piersonia-Choiromyces series, undergoes a 

 curious modification. In Piersonia bispora the hyphal palisade does not, 

 as in the other Tuberales, form asci over the whole expanse of the pas- 

 sages, but only in the innermost blind branches (Fig. 240, 4) which then 

 are widened into chambers (E. Fischer, 1908) and, in a certain sense, cor- 

 respond to the gleba chambers of the Gasteromycetes. As an exception, 

 asci may also occur in the sterile palisades, but they no longer mature. 

 If one wishes to consider the Sphaerosoma-Pachyphloeus series as tending 

 to remove the hymenium from the surface into the interior of the fructi- 

 fication and there to gradually adjust it to the whole tissue of the fructi- 

 fication, Piersonia forms a fundamental extension to this principle; the 

 hymenia are moved still more toward the interior and have entirely lost 



Fig. 241. — Genea Thwaitesii. Fructifications. ( X %] after Petch, 1907.) 



contact with the exterior of the fructification. The step from Sphaero- 

 soma to Hydnotrya, with limitation of the hymenium to the interior of 

 the fructification, has been repeated from Pachyphloeus to Piersonia; 

 here, however, the formation of asci is localized to the innermost endings 

 of the passages within the fructification. 



If one imagines the hymenial chambers of Piersonia better developed, 

 so that in cross section the hymenia form long tortuous bands and if one 

 imagines the venae externae obliterated, so that a functionless palisade 

 grows in from both sides connecting the ground tissue of both sides, one 

 has Choiromyces (Bucholtz, 1908) in which the mature hymenia lie in 

 the interior of fleshy fructifications without suggestion of passages. 



In the Genea-Genabea series, the fructifications of Genea Thwaitesii 

 (E. Fischer, 1909), as possibly those of all Tuberales, begin as small 

 hyphal tangles. Instead of the knot developing to a hollow sphere into 

 which the tramal plates grow, in Genea Thwaitesii the hymenium arises 



