196 DIVISION II.—COURSE OF DEVELOPMENT OF FUNGI. 
With respect to the more minute anatomical structure of the Tuberaceae, it may be 
further added, that the peripheral layer, known as the peridium, is usually a stout, 
thick mass of pseudo-parenchymatous tissue. The outer cell-layers are in most cases 
furnished with thickened walls corresponding in colour to the surface, which varies in 
shade from brown to black; in a few cases they are thin-walled and have their surface 
covered with spreading hairs, as in Tuber rapaeodorum, &c. Except in Stephensia, 
in which the layers of the peridium are distinctly separated from each other, the outer 
cell-layers pass gradually into the inner and these in like manner into the sterile veins 
and bands which spread between the fertile tissue, and which either show the same 
pseudo-parenchymatous structure as the peridium (Genabea) or, as in most cases, have 
their hyphae disposed in a course which follows that of the veins. Here too ascogenous 
hyphae appear in the tissue known as the fertile tissue interwoven with but strictly 
distinct from other hyphae which may be termed paraphyses. Moreover it often occurs, . 
both in Tuber and Elaphomyces, that a young ascus is placed on a knee of the hypha 
which bears it in such a manner that it seems to be borne on two small stalks, some- 
what as in Eremascus which will be described below. Tulasne has given representations 
of this phenomenon, and Dr. Errera has recently called my attention to it. It may at 
Vig 
i 

FIG. 91. Tuber rufum. asmall specimen divided in half in reflected light: the white veins 7 contain air, the dark ones v 
fluid, / the bymenial tissue. 4a thinner section through a young specimen in transmitted light ; lettering as in a, light and dark 
of the veins d. «magn. 5 times, 5 ı5 times, 

least be a question whether the development of the ascus in these cases is the same or 
similar to that of the ascus in Eremascus ; the whole subject requires investigation. 
The genera Hydnocystis, Hydnotria, and Genea are not noticed here because a 
full consideration of them would lead us too far into descriptive details, and we must 
be satisfied with remarking that they are intermediate in their whole structure between 
Tuberaceae and typical Discomycetes, especially the Pezizae; they are evidently 
closely related to both groups. 
We are indebted to Tulasne for the little that we know of the origin of the sporo- 
carps in Tuber, and from this they would appear, as has been already stated, to be 
formed inside a mycelial weft. The different regions and tissues are differentiated in 
them while they are still quite young ; the surface of specimens of Tuber mesentericum 
of the size of hemp-seed has the same structure and the same black colour as in those 
which are fully grown. Very little more is known than this. We shall have to wait for a 
complete knowledge of the history of development in these subterranean plants till we 
have succeeded in cultivating them. 
3. Onygena corvina, A. S. grows on the feathers of birds of prey and the 
mycelium which spreads in them produces stalked spherical sporocarps. The stalk is. 
7-10 mm. in length and about 1 mm. in thickness, and consists of longitudinally parallel 
