3io 



BACTERIA IN RELATION TO PLANT DISEASES. 



and thence into the stem where, in turn, the vessels of the stem are occluded and browned. 

 Subsequently if the tissues are soft enough the organism passes up and down the stem and 

 out into other leaves, always by way of the vascular system. The rapidity of movement in 

 stems depends on their texture. In hard woody stems the bacteria move with extreme 

 slowness or are entirely hemmed in; in soft juicy stems progress resembles that in the 

 petiole. In leaves which are infected from the stem (fig. 98), the entire leaf -blade may be 

 attacked almost at once and in that case may show signs of wilting. The writer has fre- 

 quently seen cabbage leaves become flabby, unjoint and fall off while the bacteria were still 

 confined to the petiole, such leaves having been infected by way of the leaf-traces as the 

 result of stem-inoculations. In these cases so many leaf-traces were involved that the leaf 

 was unable to obtain the necessary water-supply. More often some of the leaf-traces are 

 not involved and the leaf manages to function more or less imperfectly for a considerable 



time. In such a leaf a part of the veins in the 

 leaf-blade are always blackened considerably in 

 advance of the remainder and wilting may not 

 occur. The writer tried passing i per cent cosine 

 water up such petioles by transferring them to 

 the red fluid after cutting them under water. In 

 many cases the eosine only passed up the unob- 

 structed vessels, but whether failure to pass up 

 the bacterially occluded vessels was due simply 

 to the occlusion, or must be ascribed in part to 

 the destruction of the vessel-walls by the bacteria, 

 was not determined. 



Whether the first signs on the expanded 

 portion of such leaves are basal or terminal, or on 

 one side or the other of the blade, depends en- 

 tirely on which leaf-traces are entered first, dif- 

 ferent ones ramifying to different parts of the 

 leaf (Smith, Hecke). In the end, such leaves are 

 so badly affected that they unjoint and fall from 

 the stem, without, however, any signs of soft rot. 

 It is a slow dry-rot even in turnip-roots. When 

 soft rot or extensive sloughing of the parenchyma 

 intervenes, especially if it begins at the surface, 

 we may at once suspect complications due to the 

 presence of other organisms (see the soft rots). 

 When inoculations are made on the midrib of a 

 leaf, Brenner states that the bacteria pass up- 

 ward faster than downward. The writer recorded the same fact for inoculated cabbages in 

 1897, and observed it again particularly in 1906. The writer has frequently observed the 

 inoculated side of the plant to become diseased almost to the exclusion of the other side, 

 but has observed nothing suggestive of the rapid transportation of the bacteria for long 

 distances in a liquid moving stream such as we sometimes conceive to be present in the 

 vessels of a plant. In a plant inoculated on the stem under the fourth leaf Brenner observed 

 the fourth, fifth, and seventh leaves, which were on the inoculated side of the stem, to 

 contract the disease sooner than the sixth leaf, which was on the opposite side of the stem 

 (see cucumber wilt, p. 219). Brenner endeavored to force stained bacteria up cabbage- 



*FiG. 108. Small portion cf a cabbage-leaf near margin, showing how black venation due to Bacterium campestre 

 is frequently restricted for a time to angular areas formed by larger veins. This infection may have started from an 

 insect bite at a, or may have run in from a group of water-pores; at b, is another insect bite. Specimen from a cabbage 

 field in Western New York. Drawn from a photograph. About natural size. 



Fi. 108.* 



