n8 



Minnesota Plant Diseases. 



story. Again, the sac spores may be of such seldom occur- 

 rence that they have been entirely overlooked. 



Conspicuous examples of accessory spore-forms are seen in 

 the green mold growths of cheese, in the powdery mildews or 

 summer spores of the mildew fungi and in the honey-dew spores 

 of the ergot. The groups of fungi discussed in this and the 

 following chapter are subgroups of the sac fungi. (Figs. 10, 14 

 and below.) 



Yeasts and their allies (Saccharomycetes). Undoubtedly the 

 simplest of all of the sac fungi, at least as far as structure is 

 concerned, are the yeast fungi, though this simplicity is to be 

 explained by a reduction from a more complex form, due to 

 peculiar habits. The yeast plant consists of a single sphere- 

 like or somewhat elongated 

 cell, so small in size that high 

 powers of the microscope are 

 necessary for their examina- 

 tion. These cells multiply 

 rapidly by bulging out little 

 spherical "buds" which be- 

 come separated from the par- 

 ent cell and soon produce new 

 buds in their turn. A cell 

 may continue to bud off little 

 plants as long as nutrient ma- 

 terial is available. Sometimes 

 the daughter cells do not sep- 

 arate from the mother cells 

 completely but remain more 

 or less loosely attached and 

 thus false filaments or threads 

 are built up. Such are often 

 found in the scums on the sur- 

 face of yeast-containing fluids. 

 The simple method of propa- 

 gation by budding suffices the 

 yeast plant for multiplication during favorable conditions and 

 the ordinary yeast-plant-cell is often, moreover, capable of re- 

 sisting successfully very unfavorable conditions. But when 



FIG. 48. Yeast fungus cells, a. Ordinary 

 bread yeast, showing sprouting vegeta- 

 tive cells. b. Spore formation in a 

 yeast; four spores in a sac. Below are 

 shown four free spores. Highly magni- 

 fied. After Rees. 



