314 



BACTERIA IN RBI.ATION TO PLANT DISEASES. 



In experiments with Bacterium campestre on kohlrabi Hecke found that when the plants 

 were kept excessively moist under bell-jars some parts of the leaf-edge became water- 

 logged (glossy and darker colored) from an excess of water. These parts subsequently 

 died but the plants did not contract the disease from water-pores situated in such suffocated 

 areas. Consult chapter on Angular Leaf-spot of Cotton for similar observations by the 

 writer. The leaves of maize seedlings also frequently become water-logged without con- 

 tracting Stewart's disease. 



Query. — Why do the bacteria multiply so abundantly in the vessels ? 



MORBID ANATOMY. 



This organism causes no hyperplasias. After it has gained an entrance, which must 

 be ordinarily through parenchymatic tissues (epithem, etc.) the parasite is confined for 

 some time pretty closely, although not exclusively, to the vascular system and even to 

 particular leaf-traces or bundles especially to the spiral and reticulated vessels which are 

 very often filled with incalculable numbers of this organism (figs, iii, 112). When such a 

 state of occlusion exists, especially in juicy parts, the walls of the vessels are destroyed 

 in places (dissolved?) and the organism finds its way into the surrounding parenchyma, 



but never or almost never to the surface of the plant. 

 Progress through the parenchyma is slow, apparently 

 on account of its acidity. Often the intercellular 

 spaces are first occupied (figs. 113, 114); the middle 

 lamella is then dissolved (figs. 115, 116) and the 

 elements are separated and squeezed into all sorts of 

 shapes by the multiplication of the bacteria (fig. 117). 

 Subsequently the wall proper of the cell becomes 

 thinner and vaguer and finally seems to disappear 

 altogether. There may be some doubt, however, on 

 the latter point, i. e., as to the final complete solution 

 and disappearance of the cellulose. The dark band 

 at the bottom of fig. no is probably formed of com- 

 pacted cell walls crowded out of the cavity. Similar 

 cell- walls crowded out of the center of the cavity may 

 be seen in fig. 1 17 as white lines. Signified tissues are 

 not dissolved although Brenner states that they are. 

 This statement probably rests on some misinterpre- 

 tation. The spiral threads and other distinctly lignified portions of the bundle persist. It 

 is the destruction of the surrounding non-lignified tissues which gives rise to the large cavi- 

 ties in turnips and other susceptible tissues. 



The formation of cavities by this organism is very common in a number of host-plants 

 (figs. 109, 117, 118 and Vol. I, figs. 6 and 7) and all stages of the separation and destruction 

 of the cells may be studied to good advantage in turnip-roots and cabbage-leaves or cauli- 

 flower-leaves, especially the petioles. The occlusion of the vessels and the formation of 

 cavities may also be made out very satisfactorily in kohkabi and rape (figs. 107, 1 19) . These 

 cavities always begin in the vascular bundles and are occupied by the bacteria in incalcu- 

 lable numbers, scarcely an5rthing hke it being observed in the animal kingdom. Small 

 parenchyma-cavities sometimes appear around the smaller blackened veins in leaf -blades, 

 but more conspicuous ones are to be found in the fleshy midrib and petiole. As a whole the 

 parenchyma of the leaf -blade appears to be too dry or too acid for this organism. The pith 



*FiG. 112. — Bacterium campestre occupying a reticulated vessel in a turnip-root as result of a pure culture inocula- 

 tion on blades of leaf. Same vessel as shown in fig. 120 but a little farther down. Drawn from a photomicrograph. 

 X 500. 



Fig. 112.* 



