386 Dr. H. Charlton Bastian on the Origin 
as thick; in an hour and a quarter from the commencement the two 
parts of the cell-wall began to separate, and_in another quarter of an 
hour there was a distinct gap between them. By this time also all the 
protoplasm had separated from the cell-wall and formed an irregular bag 
in the centre of the cell, owing to its death under the action of the toxin, 
‘“‘ After this the changes were less rapid, and beyond a slight further 
separation of the cells, a more rotten and watery appearance of the cell- 
wall was all that could be observed.” Contiguous cells were being 
attacked in the same manner. 
“The Bacteria continued swarming around the cell-walls,” the author 
says, “and next morning (by which time the cells had been destroyed 
several layers deep) many Bacteria had come to rest in contact with the 
wall, their long axis being perpendicular to its surface, and one or two 
had the appearance of being imbedded in the wall as if in the act of 
boring their way through.” The actual penetration of the wall was 
subsequently watched on several occasions. ‘The time required varied 
with the thickness of the wall, but on an average occupied about three 
hours.” But Potter found that this Bacterium had “no power to pene- 
trate the cuticle of the mature epidermis of the turnip”; it was also 
incapable of penetrating the epidermis of mature leaves, the fully deve- 
loped cuticle being proof against the action of the enzymes excreted by 
P. destructans, though young leaves from the growing points “‘ possessing 
little or no cuticle ” were found to be vulnerable. 
The important facts made known by this research are therefore these :-— 
the vegetable cell is only capable of being penetrated when its walls are 
not thick and hard originally, and when they have been extremely 
softened by long contact with the cytase excreted by a number of Bacteria. 
The need of a conjoint attack is distinctly indicated by the author, who 
says * :—“ Very soon the number of individual Bacteria becomes 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. ... 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 Bacterium, 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 multiplication lead them to act, as it were, in concert, 
and the wall becomes softened by the cumulative action of many Bac- 
teria before the penetration of a single individual.” 
The mode in which a Mould infects and penetrates a vegetable cell is 
very similar, allowance being made for its greater size, which permits a 
single individual to do what can only be brought about by numbers of 
organisms in the case of Bacteria. This subject has been recently inves- 
tigated by Nordhausen + while studying the parasitism of Botrytis cinerea. 
I quote from Potter, who says :— 
‘“‘ 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 pene- 
traies the dead cell, and is nourished saprophytically upon it; with the 
vigour thus gained it destroys the neighbouring cells and passes from one 
* Toc. cit. p. 395. 
+ Jahrbiicher fir Wissensch. Botanik, vol. xxxiii. (1899). 
