40B 



BACTERIA. 



blood, and by so doing they are gradually enabled to overcome 

 those of a more virulent nature. 



BACTERIA PATHOGENIC TO PLANTS. Recent investigations have 

 thoroughly proved the parasitic action of Bacteria upon plant- 

 tissues, and certain diseases of plants have undoubtedly been traced 

 to these organisms, though their behaviour as vegetable parasites 

 in other cases has not been strictly denned. The following 

 examples may be quoted: 



FIG. 31x. A Cell from Turnip attacked by Pseudomonas destructans. The bacteria are 

 seen in the cell-cavity and along the track of the middle lamella. Tue cell-wall is much 

 swollen ; at a it is just beginning to separate along the middle lamella; at b the dissociation 

 is more strongly marked. 



B P. destructans with single polar flagellum. 



Pseudomonas destructans causes the White Rot of the Turnip 

 (Brassica napus). The action of this Bacterium upon the cell> 

 of its host is similar to that of certain parasitic Fungi ; it prodiices 

 a toxin which kills the protoplasm, and further secretes an enzyme 

 which has the power of dissolving the middle lamella, and causing 

 the softening and swelling of the cell-wall. By this means the 

 Bacteria rapidly invade the tissues of the host, burrowing through 

 the cell- walls and extending into the cell-cavities and intercellular 

 spaces (Fig. 34A.) ; the cell- sap escapes with the death of the cells, 



