16 EFFECT OF BLACK ROT ON TUENIPS. 



There are no bacterial pockets in the bark part of the root (phloem 

 and cortical parenchyma), forming the outer 30 to 60 rows of cells. 

 The bacterial foci, of which there are about 130 in this section, are 

 also for the most part at a considerable distance from the center of 

 the root — i. e. . in the outer part of the xylem. which, consequently, 

 we may assume either to have been the lirst portion of the root to 

 become infected or else to have been that part most readil}- attacked 

 by the organism. The cavities are all or nearly all on the rim of the 

 xylem. There are many infected bundles farther inward in the xylem, 

 but cavities are wanting in that part of the root and are still very small 

 in the outer xylem at this level, and are not clearl}' visible with this 

 magnification. Farther up the root (PI. Ill) the cavities are larger, 

 and onh" the smaller and medium-sized ones have retained their bacte- 

 rial contents intact. The sections agree, however, in that the outer 

 part of the xylem has suffered most, and in that all parts external to 

 the cambium are free from infection. 



Fig. 2 of PI. I shows a single small vessel magnified 2,000 times. 

 It is from the inner part of the root and is filled with the bacteria. It 

 corresponds to one of the vessels of PI. 11. 



PI. Ill is from a cross section in the middle swollen part of the root. 

 (Frontispiece, at point marked "2.") In comparing it with PI. II, it 

 should be remembered that the magnification is not the same. The actual 

 size of the section is shown on the lower left-hand side of the plate. 

 From the largest cavities the bacteria have diffused out into the alcohol, 

 and only the borders of these cavities are still occupied by the organ- 

 ism. That these open places are also true bacterial cavities and not due 

 to anything else is shown by an inspection of the serial sections, an open 

 cavity in one giving place to a bacterially filled cavity in another, and 

 all being free from fungi and insect injuries. Two of the largest of 

 these full cavities may be seen in the upper part of the picture. On 

 the lower left-hand side of this section is an irregular oblong cavity, 

 which was made b}" a platinum loop thrust into the tissue to remove 

 some of the organism for cultural purposes. 



PI. IV, fig. 1, represents a cross section taken from the top of the 

 root (frontispiece, point marked "1"), showing transition into stem. 

 The magnification is too small and makes evident much less than the 

 section, but suflBces for orientation, and shows that there is no surface 

 wound or large cavity. The pith and the cortical parenchyma are 

 entirely free from the bacteria. Most of the phloem is also free, but 

 bacteria are present in it at the point marked " Y."' Fig. 2 is a detail 

 from the same slide, more highly magnified. 



A study of Pis. II, III, and IV, and of corresponding longitudinal 

 sections not here represented, shows that while the vascular system of 

 the plant is very badly infested, the bulk of the tissue is still free 

 from the bacteria; i. e., at least nine-tenths of it, including all of the 



