THE BLACK ROT OF CRUCIFERS : HISTOLOGY 157 



the organisms from the water-pore region into the veins of the 

 leaf? Try microtome sections. Make preparations showing 

 the bacteria in the vessels of the leaf -blade. How soon after 

 water-pore infection can they be detected in the veins of the leaf? 

 Have you observed them forming cavities in the leaf-paren- 

 chyma around the bundles? Why do they not produce a soft 

 rot of the leaf? Does the organism ever enter the leaf through 

 ordinary stomata? Why not? How many centimeters in 

 advance of the brown stain can you trace the bacterial invasion 

 downward in the leaf ? Have you seen any indication that special 

 areas of leaf venation anastomose with special leaf-traces of the 

 petiole? 



Observe in stems of cabbage an increase of chlorophyll 

 around the diseased bundles. What causes it? (Compare with 

 leaf spots of No. VIII and with tumors of No. XIV on Paris 

 daisy). 



Where is the brown stain located? Can you reproduce it 

 in culture-media? What is its nature? Is it a humus com- 

 pound? 



Stain sections of infected leaf and stem, using nigrosin, 

 basic fuchsin, or iron hematoxylin. Make permanent prepara- 

 tions. 



Do the infected vessels contain masses of granules independ- 

 ent of the bacteria? What are these? Are they Lohnis' 

 granules? Cut the vessels longitudinally. 



What is the action of the organism on the tissues? Is 

 there a toxic action distinct from a solvent one? Is the cell- 

 wall destroyed? What then becomes of it? Consult Figs. 94 

 to 99. 



• Can you find the organism in the roots of cabbage or cauli- 

 flower? Is it commonly a root-infection? Does the organism 

 commonly ooze to the surface of attacked plants? (Compare 

 with Nos. VIII, X, XI, XII and XIII.) 



Variability. — How long does the attacked plant live? Com- 

 pare early and late infections; inoculations on slow-growing 

 pot-bound dwarfed plants with those on rapid-growing plants. 

 Study the functioning of water-pores as related to infection. 



