128 OUTLINES OF BACTERIOLOGY 



rose, ash, and olive trees. Again, in the case of those plant diseases 

 that are characterised by exudations and by rotting of the plant, 

 it is very probable that bacteria are partly or wholly responsible, 

 for there are multitudes of bacteria in the putrefying mass. Up to 

 the present, however, no casual relation has been established, for 

 more than the mere presence of an organism in a diseased part is 

 necessary to prove that the disease is caused by this organism. 



A familiar plant disease is the rot of onion. This is caused either 

 by a fungus called Botrytis, or by one called Sclerotinia. These are 

 followed by moulds, yeasts, and bacteria, which complete the work 

 of destruction, though they would be ineffective were it not for 

 the inroad previously made by Botrytis or Sclerotinia. 



A similar case can be instanced in the "leaf-curl" of the potato 

 plant. The leaves become flaccid and yellow, whilst the stem, just 

 above the soil, droops and blackens. This 

 is caused by a white mould, but the weakened 

 plant is attacked by bacteria, which cause 

 the tubers to peel in shreds, the flesh to 

 become more or less pulpy, and to emit a 

 ~^O nasty smell. The chief agent in this secondary 

 work of destruction is an anaerobic bacillus 



FIG. 81. Clostridium form of ,_,. 



bacillus. of the kind called Clostridium. (Fig. 81). 



This microbe so thoroughly decomposes the 



tuber that the latter becomes a mere bag filled with loose starch 

 grains and a foul-smelling liquid. 



The dry-rot of potatoes is due to the mixed action of various bacteria 

 and fungi. In this case the work of destruction is very slow. The 

 cell-walls of the potato are slowly destroyed by a bacillus of the clos- 

 tridium form : the destroyed tissues turn brown, the whole tuber 

 being finally reduced to a shrunken mass of a crumbly consistency. 



Of late years successful attempts have been made to investigate 

 plant diseases, on the lines followed by the investigations of animal 

 diseases. A good example is supplied us by Potter's investigation 

 of a bacterial disease which afflicts the turnip plant, and to which 

 the name "White Rot" has been given. The leaves droop and 

 become yellow in colour, the roots turn soft, and ultimately the 

 whole plant is destroyed. The organism responsible for this disease 

 is Pseudomonas destructans. It consists of short rods, each with 

 one cilium at one of the ends. The isolation of this microbe, and 

 its causal connection with the disease in question, have both been 

 successfully accomplished. The toxin which is secreted in this case 



