200 SCIENCE PROGRESS 



Laurent, in his investigations upon the potato and the causes 

 of its greater or less resistance to bacterial disease, established 

 the existence of a cytase which dissolved the middle lamella, 

 rapidly softened the cell-tissues and caused the disaggregation 

 of the cells. The organism which was the chief subject of 

 Laurent's researches, B. coli conwiunis, is very rarely capable 

 of living as a parasite upon potato tubers and other plants. It 

 was necessary for the tubers to be deprived of resistance, by 

 means of exceptional cultures, to enable the bacillus to develop 

 upon the potato. From that point its virulence was increased 

 by successive cultivations upon tubers of slight resistance, until 

 varieties at first highly resistant ended by becoming invaded 

 by the parasite. The virulence disappeared as soon as the 

 microbe ceased to be cultivated on a living tuber, cultures in 

 nutritive solutions served to suppress the aptitude of the 

 parasite and henceforward it could only be restored after special 

 preparation in alkaline solutions. In this he demonstrated a 

 complete parallel with Kissling's researches on Botrytis. 



The Bacterium causing the White Rot of turnips, which 

 was the subject of my special research, belongs to the genus 

 Pseudomonas and illustrates a very virulent form of parasite. 

 It was isolated from turnips attacked in the fields and, unlike 

 B. coli conwiwiis, flourished on nutritive media, and even after 

 many cultivations could readily be inoculated from these on to 

 pieces of living turnip, producing all the effects of the white rot 

 in about twelve hours. This organism was grown in pure 

 culture from a single bacterium and, both when living in a 

 nutrient solution and on a living turnip, was found to secrete 

 an enzyme which has the power of dissolving the middle lamella 

 and of causing the softening and swelling of the cell-wall. I 

 also demonstrated the production of oxalic acid by the bacterium 

 and that this acid acts as a toxin in plasmolysing and killing 

 the protoplasm. This proof of the secretion of a cellulose- 

 dissolving enzyme introduced a new factor and finally disposed 

 of the " impassable barrier " supposed to be off'ered by the 

 cellulose membrane to the entrance of bacteria. As the result 

 of further researches I was able to trace, by continuous obser- 

 vation, the actual penetration of the bacterium through the 

 cell-wall. The observation of the movements of the bacteria, 

 though difficult and very trying, was yet considerably furthered 

 by the difference of refractive index between the cell-wall and 



