192 



BACTERIA IN RELATION TO PLANT DISEASES. 



tubers resists well and appears to be but slowly acted upon by this organism (figs. 104, 

 105, 106). In young tubers, however, such as that one shown in figs. 83, 107, 108, there 

 may be considerable areas of the starch-bearing tissue in which no starch occurs. These 

 are most extensive on that side of the tuber longest occupied by the bacteria, and the only 

 explanations which seem to fit the case are that (1) the potato plant has withdrawn the 

 starch preparatory to the formation of a cork-barrier as Appel has described for B. phytoph- 

 thorus in potato, or that (2) excreted products of the bacteria (enzymes or other sub- 

 stances) have killed these cells or have so paralyzed their activities that the plant has not 

 been able to make use of them for the storage of starch. A very little is sometimes found 

 (figs. 105, 106). That the starch was once stored and then dissolved by diastasic action of 

 the bacteria is contrary to all that we know of its behavior, both in vitro and in older tubers. 



In the inoculated plants of the Jamestown weed (fig. 88), of the black nightshade (fig. 

 91), and Physalis philadelphica (fig. 94), there was the same enormous multiplication of the 

 bacteria in the vascular system, even at long distances from the point of inoculation, and the 

 tissues were destroyed in the same way, with the formation of extensive bacterial cavities. 



Several observers have 

 reported finding an excess of 

 crystal-sand in plants at- 

 tacked by this organism. This 

 substance occupies particular 

 cells and consists of numerous 

 discrete tetrahedral crystals 

 of calcium oxalate (fig. 88). 

 It occurs naturally in a large 

 part of the Solanaceae, the 

 amount varying with particu- 

 lar organs and with the age 

 of the parts; it is, therefore, 

 not easy to determine whether 

 there is an excess of it in dis- 

 eased tissues, but there seems 

 to be. In most of the draw- 

 ings crystal-cells are depicted 

 diagrammatically by minute 

 arrow-marks ( < < ). 



In the Solanaceae we 

 have a bicollateral bundle, the two phloems being flanked or subtended by thick-walled 

 bast-fibers, as shown very clearly in the cross-section of a potato-rhizome (fig. 84). In 

 the plants which I have examined there is also usually a large pith and considerable subepi- 

 dermal collenchymatic tissue, which is usually somewhat shrunken and torn on fixing in 

 alcohol and does not always show distinctly as collenchyma in the accompanying draw- 

 ings, but may be seen very clearly in the cross-section of a potato petiole shown in fig. 95. 

 Both phloems are frequently invaded by the bacteria and numerous cavities are produced in 

 this tissue, as shown in the figures already referred to. In later stages of the disease, cavities 

 are also common in the bark-parenchyma and in the pith. Indeed, the disorganization 

 has proceeded very far in such stems as that shown in cross-section in fig. 1 of vol. II. 



' Pig. ioi. Cross-section of small part of same potato stem as fig. 86, showing vessels occupied by Bacterium solana- 

 cearum, and surrounding tissues mostly free, except as they have flooded out of the vessels during the preparation of 

 the section. These hitter bacteria occur only on under side of section and are omitted from the drawing. Drawn from 

 a photomicrograph by the writer. Section stained with carbol-fuchsin and differentiated in 50 per cent alcohol. The 

 lignified connective tissue is represented by solid black walls and that of the vessels by fine dots. XSoo. Slide 171. 



Fig. 101. 



