42 CELL STRUCTURES 



serve as an additional protection to the organism, doubtless 

 more by selective absorption than mechanically. The 

 presence of capsules around an organism can be proved only 

 by staining the capsule. Many bacteria when stained in 

 albuminous fluids show a clear space around them which 

 appears like a capsule. It is due to the contraction of the 

 fluid away from the organism during drying. 



Metachromatic Granules. — The term "metachromatic" is 

 applied to granules which in stained preparations take a color 

 difi^erent from the protoplasm as a whole (Fig. 19). They 

 vary widely in chemical composition. Some of them are 

 glycogen, some fat droplets. Others are so-called "granu- 

 •lose" closely related to starch but probably not true starch. 

 Others are probably nuclein. Of many the chemical com- 

 position is unknown. They are also called " Babes-Ernst cor- 

 puscles" in certain bacteria (typhoid bacillus). Since they 

 frequently occur in the ends of cells the term " polar granules" 

 is also applied. Their presence is of value in the recognition 

 of but few bacteria ("Neisser granules"in diphtheria). 



Flagellum. — A flagellum is a very minute thread-like pro- 

 cess growing out from the cell wall, probably filled with a 

 strand of protoplasm. The vibrations of the flagella move 

 the organism through the liquid medium. Bacteria which 

 are thus capable of independent movement are spoken of 

 as "motile bacteria." The actual rate of movement is very 

 slight, though in proportion to the size of the organism it 

 may be considered rapid. Thus Alfred Fischer determined 

 that some organisms have a speed for short periods of about 

 40 cm. per hour. This is equivalent to a man moving more 

 than 200 miles in the same time. 



It is obvious that bacteria which can move about in a 

 liquid have an advantage in obtaining food, since they do 

 not need to wait for it to be brought to them. This advan- 

 tage is probably slight. 



An organism may have only one flagellum at the end. It 

 is then said to be monotrichic (Fig. 20) {ii6vos = alone, single; 

 rptxos = hair). This is most commonly at the front end, so 

 that the bacterium is drawn through the liquid by its motion. 

 Rarely it is at the rear end. Other bacteria may possess a 



