APPENDIX ON ASCOMYCOTA
The Sac FungiThis is the largest division of the fungi and contains some of the most highly prized of the edible species (morels and truffles). Most sac fungi, however, are not important as food sources as they are too rare, too small, too tough or poisonous. There are a number of sac fungi that cause serious damage as parasites of garden ornamental plants (e.g., powdery mildews) or agricultural crops (e.g. ergot of rye).
Sac Fungi are so-called because the sexual spores (ascospores) are produced in a sac-like mother cell (ascus). The number of spores in each sac varies with different species but is usually eight . Spore-containing sacs are produced in large numbers in a fruitbody (ascoma). It is these fruitbodies of fungi that we find in field and forest. There are three types of fruitbodies in sac fungi viz. perithecium, apothecium, and cleistothecium.
The perithecium is a tiny, flask-shaped fruitbody and the spores are produced inside. Individual perithecia are usually less than one millimetre tall. Sometimes, however, they can be produced in such abundance that they are readily seen en masse as in Nectria. Often perithecia are produced in large numbers embedded in a large fungal mass referred to as a stroma. This is the case for Xylaria polymorpha where the stroma is large (for a fungus) and gives rise to the common name of Dead Man's Fingers. If you break Xylaria in two, you will see many tiny chambers lining the outside edge of the stroma. Each one of these chambers is a perithecium. Perithecia are usually embedded in the stroma with only the neck protruding. This gives the surface of the stroma a pimpled appearance. The spores are either shot out of the pore at the tip of the neck or ooze out like toothpaste.
The apothecium is the largest and most variable of the fruitbody types. Apothecia are shaped like plates, cups, saucers or urns. Some are club-shaped or have stalked caps. The cup-, saucer- or disk- shapes are the classical apothecial forms but the fruitbodies of morels, false morels, saddle fungi and the like are also considered apothecia. The key feature of sac fungi with apothecia is that the spore mother cells (asci) are produced in a layer (hymenium) over the upper or outside facing surface of the fruitbody and are exposed to the air at maturity. A single fruitbody may produce 100's of millions of asci over its surface.
Tubers or truffles are actually modified apothecia which have become enclosed and are now subterranean and potato-like. The hymenium is highly convoluted and the spores remain trapped inside until the tuber is eaten by a rooting animal usually a rodent. The spores pass through the digestive tract unharmed and are dispersed in the droppings. To attract animals, truffles give off strong aromatic odours and even pheromnes that are sexually attractive to some animals.
In sac fungi with apothecia, the ascospores are discharged violently. When ripe, an ascus explodes and shoots its spores into the air above the fruitbody. Ascospores are so tiny as to be invisible. Sometimes, however, millions of asci fire simultaneously into the air and this cloud of 100's of millions of spores appears as "smoke." This phenomenon is called puffing. If you approach a cup fungus carefully and breathe on it gently it may reward you by puffing. The tiny spores are carried great distances by wind to new sites for growth.The cleistothecium is a tiny sphere, usually less than a millimetre in diameter and is difficult to spot without a hand lens. Thousands of sacs with their spores are produced inside the cleistothecium and these are eventually released by breakdown of the outside wall of the fruitbody. In your garden, you will often find the fruitbodies of powdery mildews, which appear as dark dots, on the undersurface of the leaves of numerous ornamental host plants. More than thirty thousand species of flowering plants are attacked by powdery mildews!
Most of the sac fungi are too small to be significant as edibles but biologically are extremely interesting as wood rotters, parasites (of insects, plants or other fungi) or as saprobes (=saprophytes) on a wide range of organic materials. Some sac fungi, however, such as the morels and truffles are amongst the most highly prized of the edible fungi and are treasured by gourmets. It is seldom reported that some morels are not as "choice" as the gourmets would allow. So, if you are disappointed by your first morel give them another try but always cook them before eating. The false morel, Gyromitra esculenta, is classified as poisonous. It is, however, a popular edible fungus eaten by many, especially in eastern Europe. While many people eat it with impunity, nevertheless, this fungus has been responsible for the deaths of hundreds of people over the years and should be avoided. It contains a toxin which elicits poisonings at different levels in different people and which can be eliminated in some preparations, hence its Jekyll and Hyde nature.