I 7 6 



BACTERIA IN RELATION TO PLANT DISEASES. 



the potato, as long brownish streaks, although the surface of these parts still appears to be 

 normal. After a time, especially in soft and rapidly growing plants, the pith and bark are 

 both involved more or less; and in the pith large cavities may arise, these being occupied 



by the bacteria and by detritus of the tissues which they 

 have disorganized (see vol. II, fig. i). Tomato plants 

 have been seen in which nearly the whole pith had been 

 converted into a watery slime. The localization of the 

 signs in the form of large, open wounds at the surface 

 of the earth (see Basal Stem-Rot) is not characteristic 

 of this disease (see pis. 24, 25). In the tomato and the 

 egg-plant, so far as observed by the writer, this is pri- 

 marily a disease of the vegetative part of the plant rather 

 than of the fruit, which is itself subject to a special dis- 

 ease (the Point-rot). The writer obtained, however, a 

 local infection in the green fruits of the tomato by inocu- 

 lation. The inoculated side as a whole ripened sooner 

 than the opposite side, but the tissue immediately border- 

 ing on the punctures remained green for a long time. 

 The bacteria multiplied considerably in the inner tissues 

 and there was a brown stain, but not what might be mis- 

 taken for a soft 

 rot, or the Point- 

 rot. The roots as 

 well as the stems 

 are subject to this 

 disease (fig. 81). 

 In good-sized, 

 stocky potato 

 plants, even when 

 the vascular sys- 

 tem is badly dis- 

 eased (browned 

 and occupied by 

 the bacteria), the 

 exterior of the 

 stem is of ten green 

 and normal in ap- 

 pearance, except 

 the leaves and ex- 



Fig. 82.' 



Fig. 83.f 



*Fic. 82. Vertical section through external part of a potato tuber from plant No. 14, which was inoculated with 

 Bacterium solanaccarum on June 15, 1896, by needle-pricks on the upper part of the stem. Material collected and fixed 

 in strong alcohol on July 27. Tuber sound externally. This represents a very early stage in the rot of the tuber. The 

 bacteria entered the tuber from the vascular system of the stem, and were still confined to the vicinity of the vascular 

 ring ( V). Below this region is the central starch-bearing parenchyma; above it is the cortical parenchyma, ending at 

 the top of the drawing in cork (potato skin) ; the oval dark bodies are starch-grains; there are very few in the vicinity of 

 the bundle, but this is not due to their destruction by the bacteria (see figs. 104, 105). The bacteria lie within and 

 between the cells, and have already eroded a considerable cavity. Drawn under the Abbe camera from material 

 which was cut on the microtome after infiltrating with paraffin. Cells shriveled by the strong alcohol. Slides 156(2 

 and 156(6. 



fFlc. 83. ^Cross-section of a small developing potato tuber diseased by Bacterium solanacearum, collected at 

 Portsmouth, Virginia, October 1905. Tuber infiltrated with paraffin and cut on the microtome; section stained with 

 carbol-fuchsin. The skin and subjacent layers are unbroken, the infection having taken place by way of the under- 

 ground stem. The unshaded parts denote absence of starch and the dark blotches the presence of the bacteria. (For 

 details at A, B. see figs. 107, 108.) The most crystal-sand cells are on the left side. There are a few starch-bearing 

 cells at X. For a detail of this part see fig. 106. Slide 349(15. 



