MOLDS AND YEASTS AND DISEASES CAUSED BY THEM 241 



insects and thus furthers the distribution of the parasite. Later 

 the myceUal mass produces sclerotia, which are masses of thick- 

 walled cells containing starch and oil together with specific poi- 

 sonous substances, and the whole becomes dry and hard with black 

 outer covering, forming the ergot grain, which is considerably 

 larger than the normal rye grain. In autumn this falls to the 

 ground and remains until spring, when numerous red stalks grow 

 out of it. Upon the swollen ends of these stalks, abundant as- 

 cospores are produced, and these serve to infect again the flowers 

 of the new crop of rye. 



This fungus is of great importance as the source of the drug, 

 ergot, and as a cause of food poisoning, ergotism, in certain coun- 

 tries. It is one example of a mold parasitic upon higher plants. 

 There are very many different species of such parasitic fungi, 

 and they are probably the best known microbic agents causing 

 diseases of plants.^ 



Saccharomyces Cerevisiae.^ — This orga,nism is the type of the 

 true yeasts. The cell is spherical or ovoid and multiplies by 

 budding. Ascospores are produced, usually four to eight in a 

 single cell. Saccharomyces cerevisicB is found widely distributed, 

 especially on fruits and in sugar-containing substances. It has 

 been used for centuries in the leavening of bread and in the al- 

 coholic fermentation. Varieties of the species are distinguished 

 by differences in physiological activity and especially in respect 

 to the amounts of alcohol which they produce. 



Material for study may be obtained from commercial com- 

 pressed yeast, which contains vegetating cells of Saccharomyces 

 mixed with other organisms including as a rule Oidium lactis and 

 various bacteria, or from commercial dried yeast in which the 

 ascospores are present. Pure cultures may be obtained by plating ' 

 this material on nutrient gelatin. Saccharomyces is found in the 

 gastric juice at times and is evidently capable of multiplying within 

 the stomach when the acidity of the gastric juice is diminished. 1 



•■ For a consideration of molds in relation to plant pathology, see Massee, Diseases 

 of cultivated plants and trees, New York, 1910. 

 , 16 



