200 



INTRODUCTION TO BOTANY 



their size are almost meaningless. When we say that average 

 rod-shaped bacteria are about Y^-Q^ hich (^^ 7 mm.) long, 

 and 23^^ inch ( T -<yV?r mm.) in thickness, we are describing 

 dimensions so small that we cannot appreciate them. It must 

 also be kept in mind that many kinds of bacteria are smaller 

 than the dimensions given above. These figures mean more 

 when we calculate the number of average bacteria that might 

 be contained in a vessel measuring a cubic inch, or when we 

 measure the thickness of a finger 

 nail and estimate how many bacte- 

 ria, piled one upon another, would 

 be required to make a column as 

 high as the finger nail is thick. 

 Such estimates will make it quite 

 clear that bacteria are small enough 

 to be everywhere and yet escape 

 our attention. Bacteria are of three 

 different forms. Most of them are 

 rod-shaped, or of the bacillus form, 

 some are spherical, and still others 

 are spiral in form (fig. 165). The 

 rod bacteria vary in length and in 

 diameter. Similarly, spherical bac- 

 teria vary in size, and some spiral 

 forms may consist of less than one 

 complete spiral, others of a dozen or more spirals. 



Little is known about the structure of these minute plants. 

 The definitely organized cell wall contains a small amount of 

 protoplasm, which sometimes forms extruding cilia (fig. 165). 

 By means of these cilia some kinds of bacteria are able to swim, 

 and some of them move with a speed (over two thousand 

 times their own length in one hour) which is remarkably 

 rapid in proportion to their small size. Definite nuclei have 

 not been seen in bacteria, but it may be that certain granular 

 fragments represent the nucleus. We are more interested, 

 however, in how bacteria live than in their structure. 



FIG. 165. A group of bacteria 

 illustrating type forms 



A, spherical bacteria gathered into 

 a colony, and B, spherical bacte- 

 ria arranged in a chain ; C, D, E, 

 and F, rod, or bacillus, bacteria, 

 E and F having cilia ; G and H, 

 spiral bacteria, the former less 

 than one complete spiral, and the 

 latter several spirals. Rearranged 

 after Jordan 



