144 PLANT PARASITES. 



2. Rod-like bacteria. Bacilli (singular, bacillus). 



3. Spiral bacteria. Spirilla (singular, spirillum). 



All straight bacteria which have one axis longer than the other are 

 called bacilli, even though the form is oval rather than rod-like. The 

 ends of bacilli may be square or 

 rounded, and in stained preparations 

 in some cases concave. 



While the cocci elongate a little 

 in preparation for fission and in 

 this condition present a slight ir- 

 regularity in the length of their 

 axes, and thus resemble bacilli, the 

 complete observation of their life FIG 72 ._ BACTERU WITH FLAf!ELLA . 

 cycle rarely permits error in the de- 

 termination of the primary group to which a given micro-organism 

 belongs. 



Some bacteria present slight modifications of the fundamental form 

 in certain phases of their growth and under various chemical and physi- 

 cal conditions. Thus some of the cocci after division are slightly flat- 

 tened on their contiguous sides biscuit-shaped; certain bacilli may 

 bulge slightly in the middle clostridium forms; others may be larger at 

 one end than at the other racket -shaped; bacilli from the same culture 

 may present considerable irregularities in breadth and especially in 

 length. The morphology may differ somewhat under different growth 

 conditions. But these slight variations in form rarely give rise to serious 

 difficulty in classification. 



When the conditions of life are unfavorable or when they are old, 

 bacteria may contain more deeply staining granules called metachromatic 

 granules. These have been regarded by some as spores, but they appear 

 to be the results of cytoplasmic degeneration. The less degenerated 

 portions retain the stain. 



Finally, when bacteria are placed under conditions unfavorable for 

 the maintenance of their life processes, and when they are dead, they 

 are often irregularly swollen and contorted or 

 may undergo partial disintegration, giving rise 

 to what are known as " involution forms. " 



While all bacteria are minute there is among 

 them considerable diversity in size, some being 

 many times larger than others. ' 



When the bacteria are about to multiply by 



FKJ. 73. BACTERIA WITH ('AP- " 



SULKS. (Pneumocomis.) fission they elongate, a dividing septum lorms, 



they become constricted at a right angle to the 



axis of elongation, and finally two independent organisms are produced. 

 The multiplication of bacteria by fission may, when the conditions are 



1 For convenience of expression microscopists have agreed to let the letter n stand 

 for the word micromillimetre, which is one-thousandth part of a millimetre This unit 

 of measure, equal to about one-twenty-five thousandth of an inch, is often called a 

 micron 



