Forms of Bacteria 16 



sometimes remain attached to each other, giving rise, 

 in time, to long chains of bacteria. 



Rod-shaped and spiral-shaped organisms (Figs. 5, 6) 

 increase only in one direction, and multiply by elongating 

 and separating into two parts. The spherical species, on 

 the other hand, are of the same diameter in length 

 and breadth, and increase in numbers by dividing in 

 either direction, lengthwise or crosswise, thus giving rise 

 to square and cubical packets. 



Irregular forms. — The three main types are not 

 always well defined. The rod-shaped bacteria may be- 

 come so short as to assume a spherical appearance. 

 The spiral forms, also, may become shortened to such 

 an extent as to disguise their true nature. Again, the 

 rods may become thic.kened in the middle and seem to 

 the eye like boat-shaped masses, designated as Clos- 

 tridia; or, they may become thickened at one end and 

 assume the shape of drum-sticks or clubs. Under cer- 

 tain conditions, the organisms may become quite irregu- 

 lar in outline, giving rise to the so-called involution 

 forms. A familiar example of irregular forms may be 

 found in the growth of the organisms in the root nodules 

 of legumes. The organism that penetrates the hair-root 

 of the plant and gives rise to the formation of tubercles, 

 is small and cylindrical in shape. Its offspring, on 

 the other hand, are much larger and different in ap- 

 pearance, for they may not only look like irregular 

 rods, but may also appear pear-shaped, club-shaped, or 

 X- or y-shaped. 



Shape. — Grouped as to shape, the spherical forms are 

 designated as cocci; the rod forms as bacilli, or bacteria. 



