[Illustration: Fig. 228. Fruit body of another mildew with dichotomous appendages. Genus microsphæra. Figs. 227-228.—Perithecia (perithecium) of two powdery mildews, showing escape of asci containing the spores from the crushed fruit bodies.] [Illustration: Fig. 229. Contact of antheridium and carpogonium (carpogonium the larger cell); beginning of fertilization.] [Illustration: Fig. 230. Disappearance of contact walls of antheridium and carpogonium, and fusion of the two nuclei.] [Illustration: Fig. 231. Fertilized egg surrounded by the enveloping threads which grow up around it. Figs. 229-231.—Fertilization in sphærotheca; one of the powdery mildews. (After Harper.)] =415. Number of spores in an ascus.=—The ascus is the most important character showing the general relationship of the members of the sac fungi. While many of the powdery mildews have a variable number of spores in an ascus, a large majority of the ascomycetes have just 8 spores in an ascus, while some have 4, others 16, and some an indefinite number. The asci in a perithecium are more variable. In some ascomycetes there is no perithecium. [Illustration: Fig. 231_a_. Edible Morel. Morchella esculenta. The asci, forming hymenium, cover the pitted surface.] =416. The black fungi.=—These are very common on dead logs, branches, leaves, etc., and may be collected in the woods at almost any season. The perithecia are often numerous, scattered or densely crowded as in Rosellinia. Sometimes they are united to form a crust which is partly formed from sterile elements as in Hypoxylon, or they form black clavate or branched bodies as in Xylaria. The black knot of the plum and cherry is also an example. The lichens are mostly ascomycetes like the black fungi or cup fungi, while a few are basidiomycetes. =417. The morels (Morchella).=—There are several species of morels which are common in early spring on damp ground. Either one of the species is suitable for use if it is desired to include this in the study. Fig. 231a illustrates the Morchella esculenta. The stem is cylindrical and stout. The fruiting portion forms the “head,” and it is deeply pitted. The entire pitted surface is covered by the asci, which are cylindrical and eight spored. A thin section may be made of a portion for study, or a small piece may be crushed under the cover glass. =418. The cup fungi.=—These fungi are common on damp ground or on rotting logs in the summer. They may be preserved in 70 per cent alcohol for study. Many of them are shaped like broad open cups or saucers. The inner surface of the cup is the fruiting surface, and is covered with the cylindrical asci, which stand side by side. A bit of the cup may be sectioned or crushed under a cover glass for study. Mushrooms (Basidiomycetes).