BACTERIA AND THEIR ALLIES 163 



It is of great importance for the proper consideration of this 

 possibility that we should have some definite knowledge as to the 

 powers possessed by such microorganisms as Bacteria and their 

 allies of penetrating the tissues of plants and animals — that is, as 

 to the means by which they are enabled to do so, as well as con- 

 cerning the time needed for such an operation. Fortunately one 

 investigation, very much to the point, can be referred to, which was 

 made by M. C. Potter in connection with a Bacterial disease of the 

 turnip.' This investigator showed that the Bacteria which cause 

 the disease secrete an enzyme (cytase) that tends to soften and 

 partially dissolve the cell-wall, and also a toxin which kills the cell. 



The important facts made known by this research are these : the 

 vegetable cell is only capable of being penetrated when its walls 

 are not thick and hard originally, and after they have been 

 extremely softened by long contact with the cytase excreted by a 

 number of Bacteria — the need of a conjoint attack being 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 Bacteria 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 investigated 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 the initial stages of germination before any trace of the 

 germ-tube can be detected. Its manner of effecting an entrance 



' " Proceed, of Royal Society," vol. Ixvii, p. 442. 



' "Jahrbiicher fiir Wissensch. Botanik," vol. xxxiii., 1899. 



