32 THE BIOLOGY OF BACTERIA 



to raise the pure culture to a temperature approaching its thermal 

 death-point. A third way of securing the same end is to place it 

 under disadvantageous external circumstances, for example in a too 

 alkaline or too acid medium. A fourth method is to pass it through 

 the tissues of an insusceptible animal. Thus we see that, whilst 

 the favourable conditions which we have considered afford full 

 scope for the growth and performance of functions of bacteria, we are 

 able .by a partial withdrawal of these, short of that ending fatally, 

 to modify the character and strength of bacteria. In future chapters 

 we shall have opportunity of observing what can be done in this 

 direction. 



Bacterial Diseases of Plants 



Eeference has been made to the associated work of higher 

 vegetable life and bacteria. The converse is also true. Just as 

 we have bacterial diseases affecting man and animals, so also plant 

 life has its bacterial diseases. "Wakker, Prillieux, Erwin Smith, 

 and others have investigated the pathogenic conditions of plants 

 due to bacteria, and though this branch of the science is in its 

 very early stages, many facts have been learned. Hyacinth disease 

 is due to a flagellated bacillus. The wilt of cucumbers and pumpkins 

 is a common disease in some districts of the world, and may 

 cause widespread injury. It is caused by a micro-organism 

 which fills the water-ducts. Wilting vines are full of the same 

 sticky germs. Desiccation and sunlight have a strong prejudicial 

 effect upon these organisms. Melon Uight must not be confused 

 with the bacterial wilt of cucumbers and melons. The blight disease 

 is caused by Plasmopara cubensis, a sporulating fungus. Bacterial 

 brown-rot of potatoes and tomatoes is another plant disease probably 

 due to a bacillus. The bacillus passes down the interior of the 

 stem into the tubers, and brown-rots them from within. There is 

 another form of brown-rot which affects cabbages. It blackens the 

 veins of the leaves, and a woody ring which is formed in the stem 

 causes the leaves to fall off. This also is due to a micro-organism, 

 which gains entrance through the water-pores of the leaf, and 

 subsequently passes into the vessels of the plants. It multiplies 

 by simple fission, and possesses a flagellum. Certain diseases of 

 Sweet Corn have been investigated by Stewart, and traced to a 

 causal bacillus possessing marked characters. Professor Potter 

 believes that white-rot of the turnip is produced by Pseudomonas 

 destructans, a liquefying, motile, aerobic bacillus. 



