200 INTRODUCTION TO BOTANY 
their size are almost meaningless. When we say that average 
rod-shaped bacteria are about zg$55 inch (g}> mm.) long, 
and zzhy, inch (ggg 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 
A nail and estimate how many bacte- 
f SB ria, piled one upon another, would 
a 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 
illustrating type forms different forms. Most of them are 
A, spherical bacteria gathered into rod-shaped, or of the bacillus form, 
Seine ae oe some are spherival, and still others 
and F, rod, or bacillus, bacteria, are spiral in form (fig. 165). The 
a head Bath (lias @ and 7 yod bacteria vary in length and in 
than one complete spiral, and the diameter. Similarly, spherical bac- 
latter several epials, Deotanged teria. vary in, 8izé; and some gpiral 
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. 
