748 FUNGI 



pieces and stir it up in water in a tea-cup. Allow the yeast to settle and 

 pour off the water. Mount in water a drop of the creamy yeast left at the 

 hottom of the cup, and examine with a high power. 



Draw a single cell and observe its cell-wall, protoplasm, and vacuoles. 



Ex. 315.- — Half fill a small teacup with water and dissolve in it a teaspoonful 

 of sugar, put in a piece of German yeast about the size of a broad-bean and 

 stand the whole in a warm place. When the yeast begins to accumulate on 

 the surface of the water, take a very minute portion and mount it in water. 

 Examine with a high power and sketch the budding or sprouting cells. 



Ex. 316. — Smear some of the creamy yeast from Ex. 314 on the cut surface 

 of a slice of potato, take and place the latter on damp blotting-paper under a 

 bell-jar. 



Keep in a warm place for a week, after which scrape off daily a little of 

 the yeast and mount in water. Examine with an J objective for ascospores. 



Ex. 317. — Make similar observations upon ordinary brewer's yeast if the 

 latter is conveniently available. 



3.' Mildews. — The term mildew in popular language is applied 

 to almost any kind of spotty discoloration brought about by 

 fungi, no matter what species the latter may be. Thus we hear 

 of mildews on leather, linen, paper, and food as well as on 

 various plants, such as the cereals, potatoes, roses, and other 

 plants, and in all these instances the spots are due to minute 

 fungi, many of which belong to totally distinct classes. 



Among botanists, however, the mildews proper constitute a well- 

 marked family of ascomycetous fungi, namely the ErysiphacecR. 

 These are all strict parasites with white, cobweb-like mycelia 

 which spread over the surface of the leaves of their hosts and 

 send short haustoria into the tissues of the latter. 



Rapid reproduction during summer is carried on by short- 

 lived, but quickly-germinating conidia, which grow in chains 

 from short, simple, erect conidiophores borne on the mycelium 

 (Fig. 254). During active vegetative growth of the fungus and 

 extensive formation of its conidia, the affected parts of the host- 

 plant appear as if covered by a mealy or chalky powder. In 

 autumn small black points arise on the mycelium; these are asco- 

 carps or perithecia containing one or more asci with eight spores. 



