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INCEPTION AND PROGRESS OF THE DISEASE. 
MANNER OF INFECTION. 
Infection may take place ina 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, 7. ¢., 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. aroideae 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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