THE FUNGI WHICH CAUSE PLANT DISEASE 



15 



«ES3iP 



Fig. 7. — Spores of bacteria showing their position 

 within the cells. After Frost & McCampbeU. 



or scattered over the whole surface, diffuse or peritrichiate. 

 They are the organs of locomotion. Within the wall is the pro- 

 toplast consisting of a peripheral layer, inner strands, imbedded 

 granules and vacuoles bearing cell sap. The existence of a nucleus 

 comparable to that in higher plants is a much controverted point. 

 Spores: Typically a bacterial spore consists of a highly refractive, 

 ovoid, walled body within the mother cell. This body possesses 

 high . resistance to ordinary stains, a great tenacity against de- 

 colorizing if it be once 

 stained, a higher resist- 

 ance against adverse tem- 

 peratures and adverse 

 conditions generally than 

 do vegetative cells, and 

 finally the ability to ger- 

 minate and thus aid in 

 perpetuating the species. 

 While the absolute num- 

 ber of bacterial species 

 that form spores is large, comparatively they are few. They are 

 most frequently met among the rod forms, and are rare among 

 the spirilla and cocci. None are known among the important 

 plant pathogens. 



In the simplest cases of spore formation, the protoplasm be- 

 comes more dense in some part of the mother cell, the remaining 

 protoplasm of the cell is drawn aroimd the denser mass, and the 



whole resulting dense region becomes 

 enclosed within a special wall. Usually 

 in this process nearly all the protoplasm 

 of the mother cell, the sporangium, is 

 used. The mother cells during spore 

 formation may remain of the normal 

 Fig. 8.— Spore formation in Vegetative size and shape; they may 



bacteria. After Fischer. ^^^^ ^^ (g subtilis) Or abandon (B. 



megatherium) the habit of thread formation. Bacteria of many 

 species become swollen at the point where the spore develops, 

 Figs. 7 and 9; be this in one end (Vibrio rugula) or in the middle 

 (B. inflatus). The swelling at the end is very common, giving rise 



