ORDER TUBERALES 237 



upon the roots of higher plants. The ascocarps vary in diameter from a 

 few milhmeters up to three or more centimeters and may be found close 

 to the surface of the soil or at a considerable depth. With no direct means 

 of bringing the ascospores into the air for distribution by air currents as 

 is the case in the Pezizales, the Tuberales depend for distribution largely 

 upon the activities of mycophagous animals, probably to a considerable 

 extent insects, but, for some species at least, ground-inhabiting rodents. 

 Thus in California the so-called ground squirrels dig out the fruiting 

 bodies, doubtless attracted by the odor diffusing up through the soil, 

 and eat them on the spot or carry them away to their burrows or other 

 hiding places. In these processes pieces of the ascocarps are scattered 

 and the ascospores find their way into the soil. 



By germination of the ascospores of one species of Tuber a conidium- 

 bearing mycelium has been obtained but this has never been grown to the 

 stage where normal ascocarps were produced. Chaze and Mestas (1939) 

 made tissue cultures of Tuber melanosporum Vitt. on various culture 

 media and obtained in pure culture an extensive mycelium. Here and 

 there in this mycelium there developed dense dark-colored masses in 

 which were produced the two-spored asci and ascospores typical of the 

 species. However, typical fruiting bodies such as occur naturally in the 

 soil were not produced. 



In 1863 Anton de Bary observed typical clamp connections, such as 

 occur in many Basidiomycetes, at the base of the asci in two species of 

 Tuber. De Ferry de la Bellone (1886) observed clamp connections on the 

 brown hyphae growing out of the ascocarps of Tuber brumale Vitt., 

 T. mesentericum Vitt., T. aestivum Vitt., and T. panniferum Tul. and 

 figured this structure for the last species. Mattirolo (1887) found such 

 clamp connections in mycelium external to but connected with the spore 

 fruits of T. lapideum Matt. Greis (1936, 1938) showed that the ascus 

 hook of T. aestivum Vitt. has the structure of a clamp connection. If it 

 is confirmed that the external mycelium in this order bears clamp con- 

 nections it must be concluded that the fungus is dicaryon in nature, as 

 is the case with the secondary phase of mycelium in the Basidiomyceteae. 



The sexual process is unknown in the Tuberales, the hypogeous habi- 

 tat of the fruiting body making it very unlikely that it would be observed 

 except by rare accident. The structure of the immature spore fruit has 

 been studied in a good many cases but for many species only the mature 

 ascocarps are known. By comparing the mature stages, taking cognizance 

 of the ontogeny of the ascocarp where it has been observed, Fischer (1896, 

 1938) and Bucholtz (1902) recognized a graded series leading from types 

 scarcely different from some of the Pezizales that have almost completely 

 subterranean apothecia to very complex forms such as those of Tuber in 

 which the apothecial nature of the ascocarp is almost entirely concealed. 



