INFECTION THROUGH WATER-PORES. 



57 



campestre diffused in sterile water (plate 2, and figs. 9, 10). The infection of the black rot 

 takes place through water-pores of the cabbage, turnip, mustard and various other plants 

 of the family Cruciferae, and the bacteria may be traced very readily from the substomatic 

 chamber down into the vascular system of the leaf until they disappear, and have been so 

 traced by the writer in a number of instances in serial sections made from properly fixed 

 and suitably infiltrated material (see vol. i, figs. 76 to 79 and 115 to 117). The infection of 

 cabbage plants through the water-pores was confirmed by Russell and Harding in 1898, 

 and was also obtained in kohlrabi by Hecke in 1902, and more recently in cabbage by 

 Brenner, one of Dr. Fischer's special students. As the writer stated in January 1898 

 (Farmers' Bulletin), the separate water-pore infections on a single large plant occasionally 

 number several hundred, while very frequently the disease with its conspicuous black 

 venation may be seen extending into the leaves of the plant from fifty or more leaf-serratures. 



If the bacterial black rot is at all prevalent in a field, infections of this sort are as common 

 and as easily observed as the plants themselves, and the bacteria may be demonstrated 

 in abundance in sections made through any of the distinctly blackened leaf-serratures, and 

 in a certain proportion of them before any stain is visible. For a time the bacteria are 

 restricted to the tissues immediately under the hydatodes and over the terminal bundles, 

 but after a few days or weeks they form a closed cavity in the heart of the leaf-tooth (fig. 11) 

 and make their way into the spiral vessels under the epithem. Once established in the 

 vascular system, they multiply with great rapidity, and their downward movement 

 through the vessels of the leaf and into the main axis of the plant is then often only a matter 

 of an additional week or two. 



Siomatal infections. — I will begin with the recently discovered black spot of the plum, 

 to which I have thrice before called attention, namely, December 1902, December 1903, 

 and December 1904, at meetings of the Society for Plant Morphology and Physiology 



*FiG. II. — Water-pore infection of cabbage by Bad. campestre. The section is parallel to the surface of the leaf 

 and passes through region of blackened leaf-tooth, the dotted part being that occupied by the bacteria. Collected at 

 Jamaica, Long Island, N. Y., July i6, 1902. Slide 220 E7. 



