Though many unsatisfactory reasons have been ascribed for the phenomenon, it has been traced by M. Grimblot to the simple fact that the filaments of the mycelium invade and destroy the roots of herbaceous vegetation. Similarly, vegetation asserts itself when the cause is removed. With young trees the truffles are usually found close to the trunk, whereas with old trees they generally appear near the periphery of the circle formed by the outer roots, as well as at a distance further removed, but usually within the shade of the tree. To what extent the humus of the soil formed by the droppings of the leaves is responsible is not stated. In many respects the subject remains, as it has always remained, a complex phenomenon that baffles the naturalist, who is usually content to refer to the truffle as an "underground fungus," or "an order of sporidiiferous fungi of subterranean habit." Perhaps the definition of Dr. C. de Ferry de la Bellone, which may be summarised as follows, is as accurate as any: "A subterraneous mushroom with a mycelium or filamentous body, from which it is developed, like the mushroom, and which requires the roots of certain trees for its formation."[52] The theory that the truffle owes its genesis to the roots of trees, or is in some mysterious manner connected with them, might be accepted as satisfactory were it not that species are also found in open places where the argument could not apply. While the roots of most kinds of oaks, both deciduous and evergreen, appear to be favourable for its generation, it has been found that in a given region the best species to propagate are those which have already produced the tuber in the locality in question, certain varieties seeming to be more liable to reproduce it than others. Climate, altitude, and exposition are also to be considered as regards the choice of the kinds selected for plantations. The arboriculturist and mycologist will be interested in the various truffle-producing oaks that may be utilised, according to the site, soil, and climatic conditions. These embrace the following species and varieties: Quercus pedunculata, Q. ped. pubescens, Q. semi-ped., Q. sessiliflora nigra, Q. nigra sessil. glabra, Q. nigra sessil. pub., Q. sessil. pub., Q. sessil. laciniata, Q. sessil. magna pubes, Q. ilex, Q. coccifera. All kinds of nut-trees are likewise favourable to its production, and may be planted almost indiscriminately. The range of T. melanosporum is broadly defined as between latitude 49° north and 40° south; the question of quality depending, like that of many other esculents, largely on climate and habitat. As in the same vineyard certain portions yield a superior wine, so on particular slopes of localities that favour the truffle a product of finer quality is obtained.