BLACK ROT OF CRUCIFEROUS PLANTS. 



315 



offers a more favorable substratum. Occasionally in cabbage and collards the entire pith 

 of a stem disappears (fig. 105) and in turnips it is not uncommon for cavities in the roots 

 to occupy a large portion of the interior (fig. 106). 



Although, as stated, parenchyma is destroyed by the wedging apart of its cells, there 

 are other ways, i. c, it is not infrequent, as shown in fig. 120 and on cross-section in Vol. I, 

 fig. 5, for non-lignified cells surrounding a vessel to be entered and filled by the bacteria 

 rather than to be crushed and crowded out of the way by external multiplication. The 

 cell-wall appears to be intact as shown in the drawings and clearly no great amount of it 

 can be dissolved. It is not easy, therefore, to make out exactly the method of entrance. 

 Probably the bacteria enter these particular cells by way of pits, the vessel being first filled 

 by the organism which then either dissolves the 

 thin separating membrane of the pit or softens 

 and ruptures it. 



Harding and Brenner both mention the 

 occasional presence in the bacterial cavities of 

 granules, which do not stain like bacteria, and 

 the origin of which is somewhat doubtful. Hecke 

 states that he found such granules in undestroyed 

 vessels just beyond the advancing margin of the 

 bacterial mass, as determined by serial sections. 

 This substance stained with magdala red but 

 did not retain the iron-haematoxylin. The 

 material was fixed in a mixture of formalin, 

 wood-vinegar and wood-alcohol. Brenner is 

 inclined to consider these granules as in the 

 nature of bassorin and derived from the decom- 

 position of the host-tissues. The writer be- 

 lieves some of them to be dead and more or less 

 disorganized bacteria, which for this reason do 

 not take stains well. Such granules occur in 

 great numbers in sugar-cane attacked by Bac- 

 terium vascularum. See also Symbiosis, page 1 1 1 . 

 They offer a good subject for further research. 



There are no gaps in the bacterial occupa- 

 tion of particular vessels and, consequently, as 

 Hecke suggests, movement of the organism in 

 the tissues, probably may be assumed to be 

 due to growth rather than to self-motility. The 

 black stain always follows the bacterial occu- 

 pation, rather than precedes it, but the bacteria 

 are rarely more than 1 or 2 cm. in advance of the 

 pigmentation, so that to a good degree absence 

 of brown stain in the vascular bundles may be 

 taken to denote absence of the bacteria. f This black or brown stain may be located 



*Fig. 1 13. Cross-section of small portion of cauliflower-petiole, showing parenchyma to left of fig. 109. Inter- 

 cellular spaces occupied by Bacterium catnpestre. The cells themselves are entirely free. Section stained with carbol 

 fuchsin. Slide 1 18-5. The bacteria entered this tissue by way of one of the bundles in which there is a large cavity. 



jRussell says "the causal organism can frequently be isolated at a point 2 or 3 inches in advance of the darkened 

 tissue." (Bull. 65, p. 23.) This I have not been able to verify and am inclined to think it is a mistake. 



Errors may easilv occur, since in one of the author's own examinations of leaves infected on the blade no brown 

 stain was detected in a fresh-cut petiole on cross-section, at a certain level, using a good hand-lens, but was plainly 

 visible to the naked eye more than 2 inches farther down (away from the point of inoculation) in a few vessels of one 

 bundle on mashing the tissues for poured plates, and was then detected on the original cross-section, the surface of 

 which had meanwhile become dry and somewhat lighter colored. These browned vessels which contained numerous 

 bacteria, were 1 1 cm. below the place of inoculation, and 6 cm. below the point where the brown stain in the vessels 

 was at first supposed to have entirely disappeared. The poured plates yielded numerous colonies of Bacterium campestre. 



Fig. 113. 



