<>J flu 7'nr/ti/i ( Brnssiea juipus). 457 



" White liot " of turnips, and the isolation of an enzyme which 

 dissolved the middle lamella and caused softening and swelling of 

 the cell-wall. The pressure of teaching has prevented my publishing 

 the complete paper sooner. 



Laurent, in his investigations upon the potato and the causes of its 

 greater or less resistance to bacterial disease, also 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. eoli fommunix, is very rarely capable of living as a parasite upon 

 potato-tubers and other plants. He states that 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 sufficed to suppress the aptitude of the parasite, 

 and henceforward it could only be restored after special preparation 

 in alkaline solutions. 



P. desfrudoMf on the contrary, 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 rot in about 

 twelve hours ; cultures both on nutritive media and on the turnip also 

 rapidly invaded the tissue of the potato. AVhether, therefore, it has 

 any existence in a saprophitic form or not, it has evidently become 

 strongly established as a parasite attacking the turnip, and probably is 

 not confined to the turnip alone. 



Wehmer has recently attempted to show that bacteria are not 

 parasitic in the case of the wet rot (Nassfiiule) of the potato, and that 

 their action is only secondary. He maintains that bacteria only attack 

 dead or unhealthy tissue, that the warmth and moisture of the damp 

 chambers impair the health of the cells, and infection is only possible 

 under conditions which renders the tissues morbid. The wet rot, 

 Wehmer says, logins with a maceration of the tissues ; between the 

 separating dead cells numerous small bubbles are to be seen and masses 

 of a small rod-like schizomycete. The initial stage is one of pectin- 

 fermentation, succeeded by cellulose fermentation. AVith these 

 processes are associated two special forms of bacteria. Wehmer's 

 description of the rotting tissues agrees with my own, but he makes 

 no mention of the enzyme nor of cultures of the bacteria. His 

 conclusions that bacteria are not parasitic cannot be accepted in view 

 of the isolation of the special enzyme by Laurent and myself, and of 

 my experiments proving the infection of sound, healthy turnips when 

 growing under perfectly natural conditions. 



