286 



BACTERIA IN REI.ATION TO PLANT DISEASES. 



vessels. The cells are separated from each other, crushed and dissolved (?) their place 

 being taken by the rapidly multiplying bacillus (fig. 62). For an earlier stage of bundle 

 infection see vol. i, fig. 9. In this way cavities arise which by fusion with other cavities 

 lead to the honey-combing and more or less complete destruction of this part of the bundle 

 and consequently to the interruption of its function, viz., the movement of water. The 

 bacteria also pass outward through the lignified tissues (by way apparently of the pits) into 

 the large pitted vessels, several to man}'- of which are often filled partly or completely before 

 there is any destruction of the phloem or of the general connective tissue of the stem. In 

 the end, the bacteria may be found also in the phloem and outside of the bundles in the 

 surrounding tissues. For an especially good example of a late stage in which the bacteria 

 have passed beyond the limits of the bundle and may be seen occupying the intercellular 

 spaces and the interior of parenchyma cells see fig. 81. By this time, however, the stem 

 begins to shrivel from loss of water, and the activities of the organism cease, so that the 

 phloem and the tissues lying between the bundles, or beyond them toward the periphery of 

 the stem, are seldom occupied to any great extent. Frequently pitted vessels at the outer 



angles of the xylem become filled in advance of those 

 in the middle. The lignified tissues are not dissolved, 

 but the thin non-lignified membrane separating the pits 

 on contiguous vessels must be ruptured or dissolved 

 by the bacteria — otherwise it is impossible to account 

 for their diffusion into the connective tissue of the 

 xylem and from one pitted vessel to another. 



It is very easy to demonstrate microscopically 

 the presence of the bacteria in the vessels, to cultivate 

 them therefrom (when the right methods are used) 

 and by means of sections made from pieces embedded 

 in paraffin to show all stages in the destruction of cells 

 and in the formation of cavities in the bundles. The 

 organism occurs also in the green fruits of cucumbers 

 and produces therein the same occlusion of vessels and 

 breaking down of neighboring cells, with the formation 

 of small bacterial cavities, as in the stem. The fruit 

 finally shrivels and the flesh sometimes has a water- 

 soaked look about the bundles, but there is no general disintegration of the parenchymatic 

 tissues, i.e., no soft rot. 



Numerous examinations under the microscope have disclosed no tendency of the cells 

 of the host-plant to enlarge or divide in the presence of the organism, nor have I detected 

 any distortions or suppressions of particular systems of tissues such as we commonly find 

 in certain other bacterial diseases. The tissues of the 



except that, as already mentioned, I have observed in t-. „ ,. _ 



by this organism, certain proliferations which, rightly ' ■ ■ ■'- ■ ■ ' 



presence in the tissues; and also in certain inoculate ^ , ^^ _. 



cork-formation in the pricked area, and a very slow multiplication of the bacteria in the 

 bundles. 



THE PARASITE. 



Bacillus tracliciphilits EFS. — The cause of this disease is a short, straight rod with 

 rounded ends (figs. 57 and 82). When growing rapidly in the plant or on culture-media it 

 commonly measures 1.2 to 2.5/x by 0.5 to o.yyu, but it may be longer or shorter or thicker or 



*FiG. 80. — Cross-section of a squash petiole, showing 12 vascular bundles occupied and destroyed by Bacillus 

 tracheiphil-its. Tissue between bundles and toward surface is free from bacteria. Inoculation was made Aug. lo, 1905 

 (Colony E, House 4), on blade of leaf by needle-pricks. From a photomicrograph. Slide 354-3. For detail see fig. Si. 



Fig. 



