Tuberacei
The structure of the order is as follows: The exterior is covered with a layer of thickened cells, which incloses a mass of mycelium. In this may be cavities lined with a layer of cylindrical asci; or the asci may be packed among the cells of the mycelium, without cavities. In each ascus is a definite number of spores (usually four or eight), which are often beautifully marked and ridged, and are usually coloured. The Tuberacei thus belong to the division Ascomycetes. While young, they are fleshy in texture; but when mature, the interior is filled with a dusty-brown mass of spores, so that, in this stage, they look much like Puff-balls (see Lycoperdon), to which, however, they are not nearly related. The term Truffle is sometimes employed to denote a second group of subterranean Fungi, of very similar appearance, named the Hypogoei; but in the latter there are no asci, and the spores are produced, singly, on the tips of outgrowths from the ends of large cells (basidia), as in Mushrooms; but the basidia line internal cavities, instead of being exposed on the surface, as in the last-named (which see). It is better to restrict the name Truffles to the Tuberacei, or to distinguish the latter as the "true Truffles." They include a considerable number of genera, of which a good many are represented in the South of England; but they become much rarer north-wards, and few reach Scotland. The number of species is comparatively small. They vary in size, from about 1/4in. to 3in. or 4in. in diameter. The genera are distinguished by the nature and markings of the outer coat (peridium), and by the forms of the asci and spores. The chief importance of Truffles arises from the fact that several species are highly esteemed as condiments, on account of the flavour that they impart to the food with which they are cooked. The edible Truffles chiefly belong to the genus Tuber, T. aestivum being almost the only species sold in English markets. In France and other European countries, several other species are also made use of, and are even more highly esteemed. The large White Truffle (Choiromyces albus) is sometimes employed, but is inferior to the genus Tuber. Owing to their concealed growth, Truffles are very apt to be overlooked; and Truffle-hunters take advantage of the keen sense of smell of dogs, or occasionally of pigs, these animals being trained to hunt over the ground, and to stop over the spots where they scent the Fungi. The plants prefer chalky soils, and there seems to be a connection between the various species and certain trees. For example, T. oestivum is usually found in the shade of Beeches, though it also grows under Birches, Chestnuts, Hazels, Hornbeams, and Oaks; and T. macrosporum and T. brumale prefer the shade of Oaks. Attempts have been made to cultivate Truffles, or, rather, to promote their growth in suitable localities. The method found most successful in France is to inclose spaces over chalky soils, and to sow them with acorns. When the Oaks are about twelve years old, Truffles are usually sufficiently abundant in the plantation to be worth working; and they continue to be produced for twenty or thirty years. It has been observed on Salisbury Plain that when plantations of Beeches are formed, alone or mixed with Firs, in a few years Truffles are to be found in them, and they continue productive for from ten to fifteen years. Experiments by watering the soil with the washings from sliced Truffles have not given decisive results.