THE ENTRANCE OF BACTERIA INTO PLANTS-THE QUESTION OF PARASITISM- 



WHAT CONSTITUTES A PARASITE. 



Among various writers there appears to be more or less confusion of ideas on the sub- 

 ject of bacterial parasitism in plants, and on the relative importance of the various 

 factors concerned in the production of disease. This has arisen partly from a confusion of 

 terms and partly from ignorance of the facts. Wehmer's argument that "bacterial decay 

 is only the last stage of the injury begun by environment, " proves too much. It applies 

 equally well to animal diseases and, if pushed to its logical extremity, ends in a 

 reductio ad ahsurdum. No one denies that the host-plant or host-animal is more or less 

 favorably disposed for the attack of a bacterium according to a variety of external circum- 

 stances influencing growth, nutrition, and vigor. Some disturbance of the normally acid 

 condition of the stomach allows the cholera vibrio or typhoid bacillus to pass undestroyed 

 into the alkaline intestine where it thrives. A sUght rawness of the throat, with the death of 

 a few cells under certain bodily conditions, allows the diphtheria or tubercle organism to 

 obtain a foothold. A wound so insignificant as to be given scarcely a second thought, affords 

 an opportunity for arthritis, septicaemia, anthrax, plague or tetanus to develop. Are these 

 and similar diseases any the less diseases due to bacteria because the entrance of the patho- 

 genic organism was favored by some wound or exudate, or other abnormal con- 

 dition of the host? By no means! Under ordinary conditions the dead cells would be 

 sloughed off, the wounds would heal, and the organism as a whole would continue in its 

 normal physiological course. The introduction of the pathogenic organism is the real 

 fundament of the situation. It begins a new train of phenomena, and by no amount of 

 argument can it be made clear to practical men that, to quote again from Wehmer, "the 

 first sort of decay," i. e., the primary lesion which allowed the bacterium to gain an 

 entrance, "is of interest practically to the exclusion of the other. " To the individual who 

 experiences it, the puncture of a pin or thrust of a knife which ends in tetanus must always 

 be of considerably more importance than one which is followed only by temporary pain or 

 inconvenience, and so of every other injury ending in a bacterial disease. 



It is not denied that general conditions of the host-plant or animal are at times espe- 

 cially favorable to the development of the disease, and at other times unfavorable. Neither 

 is it denied that the death of a few cells, by suffocation or otherwise, may afford just the 

 necessary foothold for the beginning of a destructive disease. These may be the predis- 

 posing causes, but they are not the actual cause, neither has the fact that there are pre- 

 disposing causes been generally overlooked by animal or plant pathologists, although 

 it must be admitted that they are often difficult to disentangle from a multitude of non- 

 essentials and to bring into clear relief. There are many degrees of individual 

 predisposition to parasitism, as De Bary pointed out long ago, and as every working path- 

 ologist recognizes, t The exact determination of the factors leading to special predisposition 



tAs may be seen from the following paragraph, Reinke and Berthold pointed out as long ago as 1879 that some 

 potato tubers are much more subject to bacterial rot than others: •• 



Differenzen im Widerstands-Vermogen gegen die Infection zeigen sich aber auch zwischen verschiedenen Knollen, 

 wclche vollkommen frei von Phytophthora sind. Wir haben beobachtet, dass solche Knollen, die in einerWundegeimpft 

 und unter eine Glasglocke gelegt waren, binnen zwei Tagen sich vollstandig in eine jauchige Masse umgewandelt 

 hatten, wahrend andere die Bacterien nur auf kurze Strecke in das Parenchym eindringen liessen, und dann die nass- 

 faule Stelle durch eine Korkplatte aus ihrem Organismus auszuscheiden wussten. * * * Jedenfalls miissen zwei 

 Momente zusammentreffen, ein ausseres und ein inneres, damit die Nassfaule in einer Kartoffel ihren rapiden Verlauf 

 gewinne, welcher bereits in wenig Tagen zur voUstandigcn Auflosung fiihrt; erstens inficirende Bacterien mit ihren 

 Fermenten, und zweitens Disposition des Kartoffel-Individuums. Wo sich in letzterer Hinsicht zwischen verschied- 

 enen, ausgereiften KartofFeln Unterschiede zeigen, da sind dieselben theilweise vielleicht in der Race begriindet; es 

 ware denkbar, dass die wasserreicheren, starkearmeren Sorten leichter der Zeresetzung zur Beute fallen. Doch muss 

 diese Frage durch speziell darauf gerichtete, ausgedehnte Versuchsreihen entschieden werden. 

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