RESEARCH IN PHYTOPATHOLOGY 201 



the bacteria, which enabled the course of the latter to be 

 distinctly followed. 



It is not until the protoplasm has been killed by the toxin 

 and the cell-wall very much softened by the cytase that the 

 bacteria have the power of perforating the walls and passing 

 into the cell-cavity. It would hardly be supposed that a single 

 bacterium, through its own exertions, could soften the cell-wall 

 and pierce it at one definite point after the manner of a fungus 

 germ-tube. The extreme minuteness of the bacteria and the 

 rapidity of their multiplication lead them to act, as it were, in 

 concert and the wall becomes softened by the cumulative 

 action of many bacteria before the penetration of a single 

 individual. 



The old and fully developed cuticle is apparently proof 

 against the action of the enzymes excreted by P. destnictans ; 

 but this parasite can readily effect an entrance into its host 

 through the undeveloped epidermis of young and tender 

 structures. It is incapable of manipulating the hard and tough 

 rind of the sound turnip ; but when brought into contact with 

 a wounded surface it at once flourishes as a saprophyte upon 

 the remains of the injured cells ; very soon the number of 

 bacteria becomes largely increased and the toxin and cytase 

 have sufficiently accumulated to kill the first cell. With the 

 death of its protoplasm the cell-contents are liberated and an 

 additional supply of nutriment is thus provided ; the bacteria 

 continue to multiply, cytase and toxin continue to be set free, 

 and thus each cell succumbs in turn. 



A comparison of the parasitism of Botrytis cifierea, as 

 demonstrated by the investigations of Nordhausen, presents an 

 exact parallel. He has shown that the spore of this fungus 

 excretes a powerful toxin in its initial stages of germination, 

 before any trace of the germ-tube can be detected. Its manner 

 of effecting an entrance into a host-plant is first to kill the cell 

 by the emission of the toxin ; the germ-tube then penetrates 

 the dead cell and is nourished saprophytically upon it ; with 

 the vigour thus gained it destroys the neighbouring cells and 

 passes from one to another without difficulty. The fungus 

 hypha has the power of perforating the cuticle but only in 

 young and tender structures ; old and hardened membranes 

 could only be entered when the cuticle had been injured, or 

 when it had gained strength by special saprophytic nutrition. 



