166 



BOTANY 



A perithecium broken 

 open to show the asci. 



Mildews. — Another group of fungi that are of considerable economic 

 importance is made up of the sac fungi. Such fungi are commonly called 

 mildews. Some of the most easily obtained specimens come from the lilac, 

 rose, or willow. These fungi do not penetrate the host plant to any depth, 



but cover the leaves of the host with the whitish 

 threads of the mycelium. Hence they may be killed 

 by means of applications of some fungus-killing fluid, 

 as Bordeaux mixture.^ They obtain their food from 

 the outer layer of cells in the leaf of the host. 

 These mildews produce a spore-bearing portion 

 known as a 'perithecium. When the perithecium 

 becomes broken, a number of little sacs containing 

 the spores are released. Each sac is called an 

 ascus, and the spores contained within are ascospores. 

 Each ascospore may germinate to form a new plant. 

 Among other useful plants preyed upon by this group 

 of fungi are the plum, cherry, and peach trees. (The diseases known as 

 black knot and peach curl are thus caused.) Other sac fungi are the morels 

 and truffles, the downy mildews, blue and green molds, and many other 

 forms. One important member of this group is the tiny parasite found 

 on rye and other grains, which gives us the drug ergot. 



Yeast. — Although as a group the fungi are harmful to man in the 

 economic sense, nevertheless there are some fungi that stand in a decidedly 

 helpful relationship to the human race. Chief of these are the yeast plants. 

 Yeasts are found to exist in a wild state in very many parts of the world. 

 They are found on the skins of fruits, in the soil of 

 vineyards and orchards, in cider, beer, and other 

 fluids, while they may exist in a dr.y state almost 

 any^vhere in the air around us. In a cultivated 

 state w^e find them doing our work as the agents 

 which cause the rising of bread, and the fermenta- 

 tion in beer and other alcoholic fluids. 



Size and Shape, Manner or Growth, etc. — 

 The common compressed yeast cake contains mil- 

 lions of these tiny plants, easily the smallest we have 

 yet studied. In its simplest form a yeast plant is 

 a single cell. If you shake up a bit of a compressed 

 yeast cake in a mixture of sugar and water and 

 then examine a drop of the milky fluid after twenty- 

 four hours have elapsed, it will be found to contain 

 vast numbers of yeast plants. The shape of such a plant is ovoid. Notice 

 the granular appearance of the protoplasm of which it is formed. Look 



A, yeast plant bud just 

 forming; B, bud al- 

 most ready to leave 

 parent cell. Note the 

 nucleus (A'') dividmg 

 into two parts. After 

 Sedgwick and Wilson. 



^ See Goff and Mayne, First Principles of Agriculture, page 59, for formiila of 

 Bordeaux mixture. 



