Nov. 1,1925 
Bacterial Spot of Oowpea and Lima Bean 
859 
leaf lesions shows that the invasion is intercellular and tends to be 
restricted to the mesophyll during the early stages. In fact the 
mesophyll may be rather extensively involved without any apparent 
injury to the palisade tissue. The advance invasion is in the meso- 
e layers adjacent to the lower epidermis. The cells involved 
ne a dense reddish brown and soon collapse. The lower meso¬ 
phyll layers may thus become discolored and collapsed while the 
upper mesophyll layers and the palisade layer remain apparently 
uninjured. Later the entire thickness of the lamina is killed and 
dries out at the center of the lesion. 
In cowpea seedlings vascular infection has been frequently 
observed. The organism may gain entrance to the vascular system 
from infected cotyledons, epicotyl lesions, petiole lesions, or from 
lesions on the veins of the leaf. Reddish-brown bundles were traced 
from the infected cotyledon or epicotyl down into the hypocotyl 
and up through the epicotyl into the leaf veins (pi. 4, F). In water 
mounts, the bacteria were seen to ooze from the cut ends of these 
reddish-brown spiral vessels of the epicotyl, and in cross section 
these vessels appeared to contain bacteria. The organism was 
cultured from these internally discolored vascular bundles at some 
.distance from the lesion where it had entered the host. From 
lesions on the larger veins of the seedling leaves, vascular infection 
may extend outward a distance of a centimeter or more, and in some 
cases the reddish-brown vascular elements, being in the xylem, 
are more clearly visible from the upper leaf surface. 
A microscopic examination of unstained razor sections of these 
internally discolored veins showed that the reddish-brown color 
was localized in the walls of certain spiral vessels and that this color 
was very intense, constituting an excellent stain for the walls of the 
vessels even under the higher powers of the microscope. The stain 
was also noted in the tubes of the small veinlets in the affected areas. 
Stained strips of collapsed cells also occurred in the dorsal cortex 
of the veins and in the mesophyll along the veins. 
When the freshly cut edges of leaf lesions were examined in water 
mounts, the bacteria were seen to ooze from the cut ends of the 
veins. In cross section the bacteria were noted in the intercellular 
spaces of the parenchyma of the veins but were not actually demon¬ 
strated within the spiral tubes at any distance from the leaf lesion. 
The organism very evidently shows a preference for the vein tissues 
and travels more rapidly in the veins than in the mesophyll. In 
older leaves the tendency for internal vascular invasion is not as 
marked, although linear surface lesions of considerable length often 
occur along the veins (pi. 1, A, B; pi. 2, D). 
Shallow surface lesions on the epicotyl, hypocotyl, and petioles 
were found to be limited to the outer cortical layers. The cells in 
the epidermal and cortical layers were collapsed and dense dark 
red in color. In large hypocotyl lesions there was evidence of 
deeper invasion, not in a solid front, but in the shape of ramifying 
intercellular penetration. There was marked hypertrophy of the 
cortical cells immediately beneath the epicotyl lesions, and in many 
cases rather extensive hyperplasia had resulted from the formation 
of cross walls in these enlarged cells, suggestive of an attempt to 
occlude the lesion with a cork layer. Very early infection of the 
74333—26f-5 
