194 UNICELLULAR PLANTS. 



The minuteness of bacteria is extraordinary. Many bacilli are 

 not more than .005 mm. (-g-yVo inch) in length or more than .001 

 mm. {yt^wq iiich) in breadth. Some are very much smaller. 



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

 unicellular organisms many of them have the power to pass 

 from a free-swimming {swariniiig) 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 indiyiduals or by repeated division lumjDs of 

 jelly-like consistency {zoogloea) arise. If the jelly mass takes 

 the shape of a sheet or membranous skin (as haj)23ens 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- 

 phcation 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) 



,i^"'h. 





'••f •••"y 



'At."**'* 





o 





may appear. Different names 

 are given to the resulting forms. 

 Streptococcus is a moniliform 

 or necklace-like arrangement; 

 Stajyhylococcus^ single cocci ; 

 , ^ Dijjloccoccus, cocci in jDairs; 

 (^ ^v-^ Lejytothrix^ a filament of 

 / ] ^\" — ' bacilli; Sarcina, a plate of 

 O (3 <20cci resembling a card of bis- 

 cuit, or two or more cards 



Fig. 100— Micrococci Fig. 101.— Short 

 (unstained) from hay Bacilli (un- SUperpOSed ; CtC. , CtC. 



infusion. stained) from Spores. Soiiie bacteria pro- 



hay infusion. ^ 



duce so-called sjpores [endo- 

 sjpo^^es) in the following way : The contents of the cell 



^^^»j{. 





.AN.V.- 



'->!»•.•.• 



