MICROORGANISMS AND FERMENTATIONS 5 



there are one or more spaces containing cell sap (vacuoles) . Young 

 and vigorous cells are nearly filled with protoplasm ; old or 

 starved cells have only a thin layer on the cell wall. All cells 

 possess a nucleus which, although similar to the rest of the proto- 

 plasm in composition, is of a still more complex nature. When 

 reproduction takes place, the nucleus is divided between the two 

 new cells. 



The protoplasm is bounded by a cell wall which grows thicker 

 as the cell ages. It is composed not of cellulose, as in the case of 



FIG. 8 A. Mucor Mucedo. (After Ker-ner.) 



FIG. SB. Transverse Section of a 

 Single Sporangium. (After Bre- 

 feld.) 



the higher plants, but of an allied carbohydrate which has been 

 named hemicellulose ; it becomes coated with a nitrogenous 

 substance identical with or very similar to chitin, the chief con- 

 stituent of the shells or exoskeletons of the Crustacea and the 

 insects. Mucin, a substance which forms a sticky mass with water, 

 may also be present. As these substances are typical of many of 

 the lower forms of animal life, it may be surmised that the bacteria 

 form a sort of link between animal and plant life, and this theory 

 derives some support from a consideration of their mode of life. 

 The cell wall may sometimes swell up considerably ; in cases where 



