194 UNICELLULAR PLANTS. 



The minuteness of bacteria is extraordinary. Many bacilli are 

 not more than .005 mm. (yuW inch) in length or more than .001 

 mm. Gnriinr i ncn ) m breadth. Some are very much smaller. 



Most bacteria are at some tune free forms ; but like other 

 unicellular organisms many of them have the power to pass 

 from a free-swimming (swarming) into a quiescent (resting) 

 condition. In the latter some undergo a peculiar change, in 

 which the cell-wall becomes mucilaginous, and by the aggrega- 

 tion of numerous individuals or by repeated division lumps of 

 jelly-like consistency (zooyloea) arise. If the jelly mass takes 

 the shape of a sheet or membranous skin (as happens in the 

 mother-of-vinegar), it is sometimes described as Mycoderma 

 (fungus-skin) (Fig. 102). 



Reproduction. The bacteria increase in numbers solely by 

 transverse division. Growth takes place and is followed by trans- 

 verse division of the original cell, usually into halves. Each half 

 then likewise grows and divides in its turn. In this way multi- 

 plication may go on in geometrical progression, and with almost 

 incredible rapidity. It has been stated that such repeated divi- 

 sions may follow only an hour apart, and on this basis it is easy 

 to compute the enormous numbers to which a single cell may 

 give rise in a single day. 



If separation after division is complete, strictly unicellular 

 forms arise. If actual separation is postponed, long rods, chains, 



or plates (in the case of cocci) 

 may appear. Different names 

 are given to the resulting forms. 

 Streptococcus is a moniliform 

 or necklace-like arrangement; 

 Staphylococcus, single cocci ; 

 DiploccoccuSy cocci in pairs; 

 Leptothrix, a filament of 

 bacilli ; Sarcina, a plate of 

 cocci resembling a card of bis- 

 cuit, or two or more cards 



FIG. lOO.-Micrococci FIG. lOl.-Short 

 (unstained) from hay Bacilli (un- Superposed ; etc. , etc. 



lnfU8ion ' stained) from Spores. Some bacteria pro- 



hay infusion. r 



duce so-called spores ( </<>- 

 spores) in the following way: The contents of the cell 



00 



