146 



GENERAL BIOLOGY OF MICKO-ORGANISMS 



diseases of man or other animals have been shown to be due to 

 them, although the first diseases proven to be due to micro-organ- 

 isms were those caused by certain molds. The molds possess 

 the general morphological features of plants except for the ab- 

 sence of chlorophyll. 



cx> 



Pig. 63. — Wine and beer yeasts. A. S. elHpsoideus, young and vigorous; B, S. 

 ettipsoides, (i) old, (2) dead; C, S. cerevisice, bottom yeast; D, S. cerevisice, top yeast. 

 (Original.) 



The yeasts, in general, are ovoid, specialized cells of molds, 

 belonging to several different genera. The true yeasts, genus 

 Saccharomyces, belong to the ascomycetes. They do not grow 

 out into -long filaments but remain spherical or ovoid. The cells 

 vary from 2.5 to 12 ij. in diameter. During active growth they 

 reproduce by budding, a smaller portion being pinched off from the 

 parent cell. The true yeasts also form spores inside the cell, 



