IKTEODUCTION TO CRYPTOGAMIC BOTANY. 287 



require some study to obtain an accurate knowledge of their 

 structure. The supposed spines, for instance, with which many- 

 are said to be covered, are sometimes nothing more than 

 the angles of contiguous cells. Elaphomyces is remarkable 

 for the walls of the asci being absorbed at a very early stage of 

 growth, while the hymenium dries up ; so that the cavity of 

 the plant is filled with a dusty mass of spores mixed with a 

 few threads. They were once in consequence associated with 

 the puff-balls, to which they have no affinity. 



295. The Tuberacei are perhaps improperly placed in a dis- 

 tinct division, as their characters are derived from their place of 

 growth rather than from any essential diversity of structure, 

 and they have no relation except one of analogy to the nume- 

 rous subterranean Fungi, which will come under review in a 

 future division. They have long attracted notice from their 

 mode of growth, and for the exquisite flavour of many of the 

 species, which makes them bear a large price in our markets, 

 insomuch that they are sometimes an important article of com- 

 merce. They are, perhaps, far more common than is generally 

 supposed ; but they are difficult to find, or overlooked in conse- 

 quence of their place of growth, and the aid of trained dogs 

 or other animals is necessary where large supplies are wanted. 

 Tuber cestivum is almost the only species which appears in our 

 markets ; but in France the far more highly flavoured T. mela- 

 nosporwm, the exquisite T. magnatum, and some other 

 species, are commonly consumed. Their cultivation has hitherto 

 not been attended with the same success as that of the mush- 

 room, though many attempts have been made. The matter is, 

 however, by no means beyond hope, especially since attempts 

 to make the mycelium run have been crowned with success. 



296. Truffles are for the most part found in calcareous 

 ground. In some parts of France, as in Poitou, it is simply 

 necessary, in order to their supply, to inclose a spot on the cal- 

 careous downs, sowing it with acorns. As soon as the saplings 

 attain a growth of a few years, the truffles appear, and a har- 

 vest is obtained for many years successively without further 

 pains. 



297. Truffles require a temperate but by no means a cold 



