396 Parasitism of Pseudomonas destructans {Potter). [Apr. 7, 



largely increased, each one contributes its share of toxin and cytase, 

 and in a very short time these products 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. It is not, however, until 

 the protoplasm has been killed and the cell-wall very much softened 

 that the bacteria have the power of perforating the walls and passing 

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

 rium, through its own excretions, could soften the 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 multipli- 

 cation lead them to act, as it were, in concert, and the wall becomes 

 softened by the cumulative action of many bacteria before the penetra- 

 tion of a single individual. 



A comparison of the parasitism of Botrytis cinerea as demonstrated by 

 the recent investigations of Nordhausenf 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 further 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. 



Whether in the case of P. destructans the toxin or cytase is the 

 first excretory product I cannot say ; the latter produces the first 

 visible effect, and doubtless it prepares the way for a more rapid 

 action of the toxin. But this is immaterial ; though differing, as w& 

 have seen, in detail, the behaviour of P. destructans and B. cinerea is. 

 the same in principle. The main point is established that this bac- 

 terium has the power of destroying the living cells of the turnip, 

 and, subsisting upon their dead contents, continues, to work its way 

 through the host, and it thus acts in precisely the same manner as 

 one acknowledged parasitic fungus. 



How far this kind of parasitism may be typical of bacterial 

 diseases generally remains to be proved. Another form of Pseudo- 



* A proof of the escape of the cell-sap was afforded by subjecting cells with a 

 coloured sap to the action of this bacterium, in a hanging drop, when the coloured 

 sap could be detected slowly percolating outwards through the cell-wall. 



t M. Nordhausen, " Beitrage zur Biologie parasitarer Pilze," 'Jahrbiicher fur 

 wissenschaftliche Botanik,' vol. 33, 1899. 



