(A) Saccharomyces - yeast (Rust, fig. 17 d; V & C fig. 5.9)

Yeasts consist of a single, nonmotile cell. These organisms reproduce asexually by budding in which daughter cells are formed at one or both ends of the parent cell. In sexual reproduction the entire cell becomes an ascus with either four or eight ascospores. Many of the yeasts such as this one are of considerable economic importance to humans. Others are saprophytic, and a few are pathogenic to humans.

Prepare a slide from one of the fermenting cultures and examine under high power or oil immersion.

(1) Note the shape and size of the cells. Can you estimate their size? Are the cells the same size? Explain.

(2) Look for budding cells. The small cell continues to increase in size until it reaches the size of the parent and eventually breaks free. Before becoming free, however, it might have begun to bud; thus it isn't uncommon to see chains of 2-4 cells in various stages of budding.

(B) Penicillium and Aspergillus. (Rust, fig. 17e; V & C figs. 5.15-5.17)

These two kinds of fungi are extremely common and are, in fact, laboratory "weeds". Although they are ascomycetes, only asexual stages will be examined.

(1) Examine the demonstration cultures set up for you. Is the mycelium septate? Asexual reproduction occurs with the formation of conidiospores at the tips of an erect branch called the conidiophore. Are they the same in both genera? In some ascomycetes, the tip of the conidiophore has an enlarged cell called the vesicle, to which are attached short branches called sterigmata (Greek: sterigma = support). The conidiospores can be found attached to the ends of the sterigmata. In others the sterigmata may be branched as well.

(2) What is the relationship between Penicillium and penicillin?

(C) Sordaria.

Note (using a dissecting microscope) the numerous dark bodies dispersed throughout the colony. These bodies are perithecia (V & C fig. 5.10) and contain asci and ascospores. Remove 2 or 3 perithecia and mount in water. Working quickly, locate the perithecia (using a compound microscope) and note the ejection of asci through the ostiole (pore) of the perithecium. If the pore becomes clogged with asci, apply pressure to unclog them. Focus on a single ascus. Note the inconspicuous, transparent ascus wall. How many ascospores are there? What color are they? How are they arranged in the ascus?

(D) Peziza. (Rust, fig. 17 a-c; V & C fig. 5.12)

These fungi are considered the most complex Ascomycetes because of the elaborate fleshy "fruiting body". Many of these apothecia (V & C fig. 5.10) are cup-shaped, thus the common name "cup fungi". On a prepared slide locate the hymenium, asci, ascospores, and paraphyses.