BROWN ROT OF SOLANACEAE. 



191 



frequently attain a diameter twice that of the stem a few inches above or below the pricked 

 area (pis. 28, 29). In such enlarged areas there is usually developed around the punctures 

 a considerable area of protective cork-tissue, and an excessive amount of bundle-tissue 

 is formed. 



This malady is chiefly a disease of the vascular bundles (figs. 81, 84, 90, 100, 101, 102, 

 103), but the organism is not so strictly confined to the bundles as in case of the wilt of 

 cucurbits and the brown rot of the cabbage. There is, however, the same enormous multi- 

 plication of the parasite inside of the vessels. The walls of the vessels are dissolved in their 

 thin places or else are ruptured, and cavities are produced in the surrounding tissues, the 

 bacteria being found in the in- 

 tercellular spaces but also often 

 inside of cells without one's 

 being able to make out clearly 

 how they got there (see vol. 

 II, p. 76). The parenchyma, 

 especially in soft parts, such as 

 the pith of young potato shoots 

 and tomato shoots, succumbs 

 quite readily, a large portion of 

 the interior of such stems break- 

 ing down into a soft, wet rot 

 which sometimes oozes through 

 to the surface. The bacteria 

 first invade the intercellular 

 spaces, which are generally 

 occupied quite fully in advance 

 of the complete destruction of 

 the cells. The middle lamella 

 is then dissolved. The sepa- 

 rated cells are squeezed into a 

 variety of shapes by the multi- 

 plication of the bacteria and 

 finally are crowded aside alto- 

 gether, leaving behind cavities 

 partly or fully occupied by the 

 parasite (figs. 86, 88, 91, 93, 

 98). In woody parts such as 

 stems of old potato plants or 

 tomato plants, and of well- 

 grown egg-plants, there is 

 usually a marked browning of 



the lignified tissues and an enormous multiplication of the bacteria in the vessels and the 

 adjacent parts, but there is no such general collapse as in case of soft stems. Lignified 

 tissues are not dissolved. In the potato tuber, brown-walled cavities beginning in the 

 rudimentary vascular tissue are numerous. These cavities, which are full of bacteria and 

 remnants of the broken-down tissue, gradually enlarge and coalesce until all of this part of 

 the tuber is involved. Subsequently they reach the surface (pi. 23, portions of figs. 8 and 

 10) and open up the tuber to all sorts of external destructive agents. The starch in the 



Fig. 100.* 



*Fig. 100. Longitudinal section of a potato stem, No. 5, 1896, inoculated June 1, collected June 11, showing 

 Bacterium solanacearum restricted to a single spiral vessel. Drawn from a photomicrograph. The fibers.at the left 

 polarize in the same way as the spiral thickenings. Slide 138, Vernier readings on Zeiss photomicrographic stand 

 13.4X2.0. Stained w-ith carbol-fuchsin and washed in 50 per cent alcohol. X210. 



