i 5 6 TEXT-BOOK OF FUNGI 



the resistance. Now, in the cases described above, in 

 which species of plants were rendered susceptible to 

 biologic forms which are incapable under ordinary condi- 

 tions of infecting them, the cultural methods adopted 

 consisted in affecting the vitality of the leaf either by a 

 mechanical injury or by the action of anaesthetics or heat. 

 When the vitality l of the cell becomes affected through 

 the injury to the leaf, it may be assumed that either the 

 protective enzymes or similar substances normally present 

 are destroyed, or become weakened, or the production of 

 them by the protoplasm is interfered with, in the cells in 

 the neighbourhood of the injury, with the result that other 

 biologic forms are now able to cause infection, since they 

 no longer meet with any special substance capable of stop- 

 ping their growth. It is pointed out that these cases of 

 the loss of immunity, .brought about by causes which affect 

 the vitality of the leaf, find their exact parallel in the 

 recorded instances of induced susceptibility in animals to 

 certain bacterial diseases. The decrease of vitality caused 

 by fatigue, action of drugs, abnormal food, or environment, 

 has been proved to induce susceptibility to certain bacteria 

 in the case of an animal which had proved to be immune 

 under normal circumstances. 



Salmon suggests that in nature injuries to leaves, having 

 the same effect as those caused artificially in the experi- 

 ments mentioned above, may be caused by animals, hail, 

 storms of wind, etc., and mentions cases where, apparently, 



1 The author remarks : ' Until our knowledge of the physiology of the 

 cell has progressed further, it is necessary to use the general term vitality 

 to express the sum of the individual physiological processes at work in the 

 cell. External factors which affect the normal balance in the working of 

 the individual physiological processes may be said to increase or decrease 

 the vitality of the plant. ' 



