CHAPTER XV 



BACTERIA IN RELATION TO DISEASE 



1. Diseases of Plants ; Harmless ' Messmates ' in the Human Body ; 

 Pathogenic Bacteria ; Points of Attack and Sources of Infection. 



With the exception of the root-nodules of Leguminosae, we know of 

 no single instance where bacteria invade the closed, living cells of plants. 

 The sole and only channels of communication between the interior of the 

 plant and the outside world are the stomata, and these open into a closed 

 system of air-filled intercellular spaces which are shut off from the cells 

 themselves. If bacterial spores are blown by the wind, or washed by 

 the rain into the stomata, they can do no damage, for they can get nowhere 

 but into the intercellular spaces where they find nothing but moist air. 

 Without nutriment, bacterial spores cannot germinate, and nutriment is not 

 present in any form. Even if such microbes as can dissolve cellulose 

 (methane bacteria) drift into the air passages of a plant, they find no food, 

 and are therefore unable to injure the cell-walls. The only organisms 

 whose entrance is accompanied by any danger are those whose spores con- 

 tain sufficient reserve nutriment to enable them to germinate in pure water, 

 to grow at first without food, and open the attack on the cell-wall at their 

 own expense. Such are spores of the parasitic fungi. They contain reserve 

 food-stuffs, and protrude a hyphal filament which pierces the epidermis 

 (potato disease), or pushes its way through one of the stomata into the inter- 

 cellular spaces (rusts), and from thence into the cells themselves. The hyphae 

 of the fungus dissolve the cell-membrane and send in haustoria, or proliferate 

 in mycelial masses in the cell-cavity. 



Bacteria are totally unable to act in this way, and the uninjured plant 

 is impregnable to their attacks. Even the injured plant offers them nourish- 

 ment only in the exposed and opened cells of the wound-surface, and this 

 source is soon shut off by the development of an impenetrable layer of cork 

 below the wound preventing any further exudation of fluids. Thus the 



