BLACK ROT OF CRUCIFEROUS PLANTS. 



319 



steamed in water the formation of the brown pigment was very decided, finally approxi- 

 mating bistre or mummy brown. Harding observed a similar prompt browning in cultures 

 made on white winter radish and on cauliflower. Non-cruciferous substrata so far as tested 

 do not give any such deep brown pigmentation. 



Colonies of this organism on agar or gelatin are smooth, wet-shining, pale yellow, round, 

 flat to convex (Harding), thinning out to a distinct entire margin (fig. 128). The surface 

 colonies at first are often quite pale, becoming yellower with age. Buried colonies do not 

 develop speedily, neither is the surface growth very rapid ; Harding says rapid at 28 C. 

 Feathery X-shaped crystals of ammonium magnesium phosphate are formed after some 

 days in cultures on peptonized beef-broth-agar (fig. 128). On +15 nutrient agar after 

 a time a white chemical halo forms 

 around the colony, streak, or "nail 

 head" of the stab. This appears to 

 be common, however, to various 

 species of Bacterium. So also are 

 the crystals. 



On meat-extract-peptone gela- 

 tin, Hecke describes the young col- 

 onies as small, cloudy, colorless, 

 circular drops which became plainly 

 yellow with age and feebly zoned 

 concentrically, the gelatin liquefying 

 slowly. At his room temperature 

 (15 C. ?) the growth on this medium 

 was slow, the surface colonies on a 

 plate 15 days old measuring only 

 about 3 mm. in diameter in a 

 liquefied zone 7 mm. in diameter. 

 On neutralized kohlrabi-gelatin simi- 

 lar results were obtained. On the 

 contrary, in non-neutralized kohl- 

 rabi-gelatin growth was extraordi- 

 narily slow, so that the colonies were 

 not visible until after the tenth day. 



In my -f-io nutrient gelatin, in 

 rather thin sowings in Petri-dish 

 poured-plates, at the end of 7 days, at 



10 to 20 C., the surface colonies of Bacterium campestre under the Zeiss 16 rnm. objective 

 and 12 ocular were small, circular, and homogeneous (fine granular), with entire margins; 

 the buried colonies were globose-lobulated and less than i mm. in diameter. 



Bacterium campestre isolated by the writer from a cabbage leaf in June, 1908, on a thin- 

 sown agar plate (10 colonies) gave thin, flat, pale yellow, circular surface colonies with a 

 slight tendency to rings but no other distinct naked-eye structure. At the end of 4 days 

 at 28 to 30 C., the largest colonies were 8 to 10 mm. in diameter, the surface smooth and 

 wet-shining. With Zeiss 16 mm. apo., and 12 ocular the margin of the colonies showed no 

 special characteristics. Under this magnification the colonies were uniformly fine granular 

 except the extreme margin which was a little paler and nearly amorphous in structure. In 

 a plate of the same lot containing about 100 colonies, the surface colonies on the fourth day 



119.* 



*Fio. 119. Cross-section of a rape petiole attacked by Bacterium campestre. Plant inoculated Dec. 19, 1896. 

 Leaf fixed in alcohol Jan. 9, 1897. Bacteria restricted to the bundles, i. e., to heavily shaded parts, except on right side 

 near periphery where intercellular (shaded) spaces are occupied. Drawn from a photomicrograph made with a planar 

 lens, from a stained microtome (paraffin infiltrated) section. Actual diameter about 2 mm. Slide 107 A. 



