PLANT PARASITES. 



145 



? 6 



FIG. 74. GROWTH AGGREGATES OF BACTERIA. 



1. Diplococcus: 2, streptococcus; 3, merismopedia; 

 4, diplobacillus ; 5, streptobacilli ; 6. curved bacteria 

 forming chains. 



favorable, occur so rapidly as to give rise within a few hours to an enor- 

 mous number of new individuals. 



In many cases, the new individuals thus developed fall apart in a 

 form identical with that of the parent cell. In some species, on the 



other hand, the new-formed indi- 

 viduals are prone to cling together 

 with greater or less tenacity, thus 

 giving rise to growth aggregates 

 which are more or less character- 

 istic (Fig. 74). Thus among the 

 cocci there are those in which a 

 large part of the new individuals 

 cling together in pairs. These 

 forms are called diplococci. In 

 others the pairs cling together in 

 longer aggregates or chains. Such 

 are called streptococci. 



A similar occurrence in the 

 bacilli gives rise to diplobacilli and 

 streptobacilli. Some of the spiral 

 forms are due to the close junction 

 end to end of oppositely curvea 



segments. Certain long thread-like micro-organisms closely allied to the 

 bacteria are called leptothrix. 



Certain cocci divide in two directions at right angles to each other, 

 giving rise to four cocci clinging together and lying in the same plane. 

 These are called tetrads or merismopedia. 



Finally cocci may divide along three planes at right angles to each 

 other, giving rise to cuboidal packets of eight germs or some multiple of 

 this such growth groups are called sarcina ' (Fig. 75). 



There is a family of filamentous or branching organisms which are 

 often spoken of as polymorphous or higher bacteria, some of which may 

 indeed be links between the bacteria and higher 

 plant forms. Filamentous bacteria are more or 

 less distinctly segmented, and the segments may be 

 enclosed in a common sheath. The modes of re- 

 production and a certain specialization of function 

 in different parts of the filaments which is fre- 

 quently present indicate affiliations with higher 

 forms. By this specialization of function is meant 

 the attachment of the threads at one end to the 

 substance on which they grow, and the formation 

 at the free ends of structures which appear to be 

 concerned in the reproduction. Several groups of these organisms have 

 been named, but few of them have as yet been adequately studied. The 



1 Bacteria in masses embedded in and held together by a more or less abundant 

 homogeneous material which they elaborate are called zooglcea. 

 W 



FIG. 75. SARCINA. 



Showing growth aggregates 

 in cuboidal masses. 



