INCEPTION AND PROGRESS OF THE DISEASE. 



MANNER OF INFECTION. 



Infection may take place in a number of ways either through natural openings or by 

 way of wounds. These ways will now be considered, beginning with the most direct 

 method. 



WOUND INFECTIONS. 



The wounded surface, even though small, affords in case of many plant-organs a very 

 suitable soil for the right sort of a bacterium. Here it first makes a little growth at the 

 expense of the extruded cell-contents, i. c, it multiplies first of all in the dead tissues of the 

 wound. If the lodged organism is not a facultative parasite, growth in the wound either 

 does not occur or ceases very soon, and no disease is induced. If, on the contrary, the 

 organism is a wound-parasite, it does not remain confined to the original wound very long. 

 In such cases, growth is more vigorous, and this presumably sets up osmotic changes deter- 

 mining a movement of the plant-juices toward the wound. A protective cork-layer is not 

 formed under the wound, or is formed only imperfectly, and through the intercellular spaces 

 and the neighboring vessels there is an open passage way into the depths of the tissues, a 

 way which the parasite is not slow to make use of. Enzyms, toxines, acids and various 

 by-products of the bacterial growth also undoubtedly play their part, weakening the cells 

 of the host or destroying them outright. With increasing supplies of food, and a nidus 

 rendered suitably alkaline by their own excretions, the bacteria multiply more and more, 

 obstructing some tissues and dissolving, displacing, and crushing others. The tissues are 

 poisoned more and more by absorption of the continually increasing quantity of bacterial 

 by-products, cells are separated, cell-walls are softened or dissolved, protoplasm, amids, 

 acids, starch, and sugars are consumed. Beginning, therefore, with a tiny superficial nidus 

 in an open wound, a facultative parasite gradually burrows its way into the deeper tissues, 

 forming closed cavities or open wounds, and finally destroying the entire plant or limiting 

 its operations to special organs, as the case may be. Such is the impression one gets from a 

 study of wound-infections. 



The action of such a bacterium may be slow or rapid, depending on its own habits, 

 on the degree of resistance or susceptibility of the host-plant, and finally on whether the 

 surrounding conditions, such as temperature, light, food-supply, and water-supply are 

 most favorable to the host-plant in its opposition or to the parasite in its attack. 



The susceptibility to a given disease varies greatly in different races of the same plant, 

 and also from individual to individual, if for convenience one may be allowed the use of this 

 word in speaking of plants. In most cases the reason for this difference in susceptibility 

 is unknown. 



In my account of particular diseases I shall discuss fully the manner of infection, and 

 desire here to make only a brief statement. 



Bacillus carotovorus Jones and B. aroidcae Townsend are very good examples of wound- 

 parasites. We do not know that they ever enter the plant except through wounds. In dry 

 tissues they make only a slow progress, but in juicy tissues, at suitable temperatures, they 

 make an extremely rapid growth, and the destruction of the host is correspondingly great. 

 A single needle-prick introducing either of these organisms into the fleshy tissues of a large 

 green cucumber is sufficient to cause the whole interior to break down into a soft watery 

 mass of disintegrated cells in the course of one or two weeks. The first organism, introduced 



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