138 BACTERIA IN RELATION TO DISEASE 



wound does not remain moist ; the injured cells shrink and dry up, and the 

 entrance of bacteria is then as difficult as in the case of an uninjured plant. 

 Infected wounds are therefore dangers that have no existence for plants, and 

 the spread of a lesion to other parts is not possible. 



These considerations enable us to prophesy with considerable certainty 

 what will be the result of an injection into vegetable tissues of bacteria, even 

 of a species pathogenic for animals : no development in the intercellular 

 spaces, and but slight temporary growth on moist wounded surfaces, are 

 what we should expect, and what experiments show to actually occur (113). 



Notwithstanding these well-known facts, there appear time after time 

 descriptions (generally insufficient enough) of new 'bacterial' diseases in 

 plants. Bacteria, it is true, are often found in diseased plants in enormous 

 numbers, but they are living metatrophically only, living on tissues that 

 have already been disintegrated and decayed by parasitic fungi. The 

 bacteria assist these subsequently in their work of destruction no doubt, and 

 modify perhaps more or less the character of the disease. But, leaving 

 aside for the present injuries received from 'frost or insects, the first attack 

 on the plant is always made by fungi, which are also very harmful in causing 

 the spread of accidental wounds. All the cases of so-called ' bacteriosis' in 

 plants, from the 'gomtnose bacillaire' of the vine down to the ' schorf of 

 the potato, are primarily diseases of non-bacterial origin in which the 

 bacteria are present merely as accidental invaders (114). 



As metatrophic organisms, bacteria occur plentifully upon the leaves of 

 insectivorous plants such as Pinguicula, Drosera, and Nepenthes. The insect- 

 catching orgahs are necessarily open to the surrounding world, and bacteria 

 are blown on to them by wind, and brought by insects and other animals. 

 The half-digested remnants of captured organisms furnish a supply of rich 

 nutriment to the bacteria, and it would be extraordinary indeed if they did 

 not flourish and multiply in such places. Here again attempts have been 

 made to demonstrate a ' symbiotic ' relationship between the plant and the 

 bacteria, whose peptonizing powers were supposed to be of service in dis- 

 solving the captured insects. Careful investigation has shown these sup- 

 positions to be incorrect. In the case of Lathraea, whose leaf-chambers 

 contain hairs often thickly covered with bacteria, it is not certain whether 

 these are metatrophic or not (115). 



Among the lower animals bacterial diseases have been as yet but 

 little investigated, although they are doubtless very common. The silk- 

 worm diseases discovered by Pasteur, and the so-called ' Foul-brood ' of 

 bees, are well-known examples. Recently a Bacterium ranicidum has been 

 described, pathogenic for frogs and fishes (116). 



The chief interest as regards disease-producing bacteria is, of course, 

 concentrated on those associated with man and the higher mammalia. 

 The symptomatic characters of bacterial diseases being similar in man 



