In _Phacidiacei_, the substance is hard and leathery, intermediate between the fleshy _Elvellacei_ and the more horny of the _Sphæriacei_. The perithecia are either orbicular or elongated, and the hymenium soon becomes exposed. In some instances, there is a close affinity with the _Elvellacei_, the exposed hymenium being similar in structure, but in all the disc is at first closed. In orbicular forms, the fissure takes place in a stellate manner from the centre, and the teeth are reflexed. In the _Hysteriacei_, where the perithecia are elongated, the fissure takes place throughout their length. As a rule, the sporidia are more elongated, more commonly septate, and more usually coloured, than in _Elvellacei_. Only a few solitary instances occur of individual species that are parasitic on living plants. [Illustration: FIG. 42.--_Sphæria aquila._] In the _Sphæriacei_, the substance of the stroma (when present) and of the perithecia is variable, being between fleshy and waxy in _Nectriei_, and tough, horny, sometimes brittle, in _Hypoxylon_. A perithecium, or cell excavated in the stroma which fulfils the functions of a perithecium, is always present. The hymenium lines the inner walls of the perithecium, and forms a gelatinous nucleus, consisting of asci and paraphyses. When fully mature, the asci are ruptured and the sporidia escape by a pore which occupies the apex of the perithecium. Sometimes the perithecia are solitary or scattered, and sometimes gregarious, whilst in other instances they are closely aggregated and immersed in a stroma of variable size and form. Conidia, spermatia, pycnidia, &c., have been traced to and associated with some species, but the history of others is still obscure. Many of the coniomycetous forms grouped under the _Sphæronemei_ are probably conditions of the _Sphæriacei_, as are also the _Melanconiei_, and some of the _Hyphomycetes_. A very common fungus, for instance, which is abundant on sticks and twigs, forming rosy or reddish pustules the size of a millet seed, formerly named _Tubercularia vulgaris_, is known to be the conidia-bearing stroma of the sphæriaceous fungus, _Nectria cinnabarina_;[T] and so with many others. The following are the technical characters of the family:-- _Fruit consisting of sporidia, mostly definite, contained in asci, springing from a naked or enclosed stratum of fructifying cells and forming a hymenium or nucleus_ = ASCOMYCETES. If the characters of the different families are borne in mind, there will be but little difficulty in assigning any fungus to the order to which it belongs by means of the foregoing remarks. For more minute information, and for analytical tables of the families, orders, and genera, we must refer the student to some special systematic work, which will present fewer difficulties, if he keeps in mind the distinctive features of the families.[U]