SPHÆRIACEI.--In this group there is considerable variation, within certain limits. It contains an immense number of species, and these are daily being augmented. The general feature in all is the presence of a perithecium, which contains and encloses the hymenium, and at length opening by a pore or ostiolum at the apex. In some the perithecia are simple, in others compound; in some immersed in a stroma, in others free; in some fleshy or waxy, in others carbonaceous, and in others membranaceous. But in all there is this important difference from the Ascomycetes we have already had under consideration, that the hymenium is never exposed. The perithecium consists usually of an external layer of cellular structure, which is either smooth or hairy, usually blackish, and an internal stratum of less compact cells, which give rise to the hymenium. [Illustration: FIG. 35.--Perithecium of _Sphæria_ and Section.] As in the _Discomycetes_, the hymenium consists of asci, paraphyses, and mucilage, but the whole forms a less compact and more gelatinous mass within the perithecium. The formation and growth of the asci and sporidia differ little from what we have described, and when mature the asci dehisce, and the sporidia alone are ejected from the ostiolum. We are not aware that operculate asci have yet been detected. It has been shown in some instances, and suspected in others, that certain moulds, formerly classed with _Mucedines_ and _Dematiei_, especially in the genus _Helminthosporium_, bear the conidia of species of _Sphæria_, so that this may be regarded as one form of fruit. Perithecia, very similar externally to those of _Sphæria_, but containing spores borne on slender pedicels and not enclosed in asci, have had their relations to certain species of _Sphæria_ indicated, and these are no longer regarded so much as species of _Hendersonia_ or _Diplodia_ as the pycnidia of _Sphæria_. Other and more minute perithecia, containing minute, slender stylospores in great numbers, formerly classed with _Aposphæria_, _Phoma_, &c., but are now recognized as spermogonia containing the spermatia of _Sphæriæ_. How these influence each other, when and under what circumstances the spermatia are instrumental in impregnation of the sporidia, is still matter of mystery. It is clear, however, that in all these conidia, macrospores, microspores, and some spermatia, or by whatever names they may be called, there exists a power of germination. Tulasne has indicated in some instances five or six forms of fruit as belonging to one fungus, of which the highest and most perfect condition is a species of _Sphæria_. [Illustration: FIG. 36.--_Uncinula adunca._]